INTRODUCTION

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For any dividing cell it is essential to maintain the integrity of the genome ensuring that DNA replication occurs once per cell cycle and that identical chromosomal copies are distributed equally to two daughter cells at mitosis. When cells are subjected to conditions that interfere with DNA replication or spindle assembly or that cause damage to DNA, cell-cycle progression halts, permitting cell cycle phase completion or DNA repair. These control mechanisms are referred to as "checkpoints" (reviewed in Refs. 1-3). The loss of checkpoint control in mammalian cells, most notably through the inactivation of p53, results in genomic instability leading to the amplification, rearrangement, or loss of chromosomes, events that commonly occur in cancer cells (2, 4).

Much of our current knowledge about checkpoint control has been obtained from studies using budding (Saccharomyces cerevisiae) and fission (Schizosaccharomyces pombe) yeast (reviewed in Ref. 5). In the fission yeast at least two distinct checkpoint pathways have been identified: the DNA replication-dependent checkpoint pathway that ensures that S phase is completed before M phase is initiated and the DNA damage-dependent checkpoint pathway that acts to halt the cell cycle when genomic integrity is compromised (5). The products of six genes, rad1⁺, rad3⁺, rad9⁺, rad17⁺, rad26⁺, and hus1⁺, have been identified in S. pombe as essential components of both checkpoint pathways (5). In S. cerevisiae the two checkpoint pathways are genetically separable; four gene products, RAD9, RAD17, RAD24, and MEC3, have been identified as essential for the DNA damage-dependent checkpoint pathway, and three gene products, POL2, RFC5, and DPB11, are essential for the DNA replication-dependent checkpoint pathway (reviewed in Refs. 3-5). Several of the S. pombe checkpoint genes have structural homologues in the budding yeast and further conservation across eukaryotes has recently been demonstrated with the cloning of two human homologues of S. pombe rad3⁺, ATM (ataxia telangiectasia mutated) (6) and ATR (ataxia telangiectasia and rad3⁺ related) (7, 8), and a human homologue of S. pombe rad9⁺, Hrad9 (9).

The identification and characterization of human homologues of yeast checkpoint genes provides clear evidence that at least some checkpoint pathways are conserved between mammals and yeast. Currently little is known about the biochemistry of checkpoint control. The genetic data in yeast suggest that a complex of proteins mediates the monitoring of replication-specific structures and damaged DNA (10). Furthermore, recent biochemical studies in S. pombe and humans suggest that the cell cycle arrest in response to DNA damage is brought about by the activation of a signal transduction pathway involving the putative protein kinases ATM/ATR and Hchk1, resulting in inhibitory phosphorylation of Cdc25 and subsequent stabilization of the inhibitory Tyr¹⁵ phosphorylation of Cdc2 (7, 11-14).

Although a great deal of progress has been made in identifying the kinase components of the signal transduction pathway mediating cell cycle arrest (6-8, 12), there has been little progress in identifying the sensing mechanisms that activate the checkpoint pathways. The S. pombe rad1⁺ gene acts early in the checkpoint pathway (15) and presumably plays a role in the transmission of information regarding the state of the genome to the checkpoint control mechanism. The rad1-1 mutant is extremely sensitive to all DNA damaging agents (15, 16) and fails to invoke a G₂ arrest in response to ionizing radiation yet is DNA repair-competent, indicating a loss in the ability to recognize the presence of damaged DNA (15). The rad1-1 mutant is also sensitive to hydroxyurea, indicating an uncoupling between the completion of S phase and entry into mitosis (15), and is synthetically lethal when combined with mitosis-promoting mutations such as wee1-50 (15). The S. pombe rad1⁺ gene has been cloned (17), and the predicted amino acid sequence shows significant sequence similarity to the Ustilago maydis protein Rec1 (18) and S. cerevisiae RAD17 (20). Mutations in these genes lead to cell cycle checkpoint defects and other phenotypes associated with alterations in DNA repair and recombination (19, 20).

Purified Rec1 protein has been shown to have a 3'  5' exonuclease activity (21), suggesting that Rec1/Rad1/RAD17 may be involved in modifying DNA damage (22, 23). Studies of Rec1 truncation mutants have demonstrated that the nuclease activity is located in the N-terminal half of the protein. However the Rec1-dependent checkpoint function requires the C-terminal region of the protein and does not require the exonuclease activity (24, 25). In addition, Rec1 mutants that lack exonuclease activity have a 100-fold higher rate of spontaneous mutation, indicating that the exonuclease function may be required for mismatch repair (24). Thus it would appear that the Rec1 protein is bifunctional, having an N-terminal exonuclease domain and a C-terminal checkpoint domain.

In this report we describe the cloning and characterization of a novel human cDNA, designated Hrad1, which is highly similar to the S. pombe rad1⁺ checkpoint gene. We also report the cloning of a mouse homologue designated Mrad1. We show that Hrad1 is subject to alternative splicing giving rise to two potential open reading frames (ORFs)¹ coding for a full-length and a truncated form of Hrad1, designated Hrad1A and Hrad1B, respectively. We have expressed in Escherichia. coli and affinity purified N-terminal polyhistidine-tagged fusions of Hrad1A and Hrad1B and show that Hrad1A but not Hrad1B has 3'  5' exonuclease activity. Hrad1B corresponds to the C-terminal domain of Rec1 that has been implicated solely in checkpoint function. Thus we can speculate that Hrad1A plays a role in the recognition and processing of DNA ends and also in checkpoint function, whereas the role of Hrad1B may be restricted to checkpoint control.

	EXPERIMENTAL PROCEDURES

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Cloning and Sequencing of Human and Mouse rad1-- A search for sequences similar to S. pombe rad1⁺ was carried out using the TBLASTN program (26) against the GenBank^TM data base. Deduced amino acid sequences were aligned using the CLUSTALW program, and similarity was determined with a blosum62 amino acid substitution matrix. An expressed sequence tag (EST) clone with significant sequence similarity to S. pombe rad1⁺ was identified and obtained from the Integrated Molecular Analysis of Genomes and their Expression (I.M.A.G.E.) Consortium.

Northern Blot Analysis-- Two multiple human tissue Northern blots (CLONTECH) and a human cancer cell line Northern blot (CLONTECH) were hybridized with a full-length Hrad1B cDNA probe, labeled with [-³²P]dCTP by random hexamer priming with the Prime-a-Gene Kit (Promega). The blots were hybridized in ExpressHyb solution according to the manufacturer's instructions (CLONTECH) and washed at high stringency (0.1 × SSC, 0.1% SDS, 50 °C, 2 × 20 min) and exposed to Kodak X-Omat autoradiography film with intensifying screens at 70 °C. The blots were then rehybridized with a human -actin probe (CLONTECH) labeled with [-³²P]dCTP as described above to verify the intactness of the RNA samples and confirm the presence of comparable amounts of RNA across lanes.

S. pombe Strains, Culture, and Plasmids-- S. pombe was cultured by standard techniques (27). The genotypes of the strains used are as follows: APY002, h⁺ leu1-32 ura4-D18 ade6-M216, and GBY190, h⁺ rad1::ura4⁺, leu1-32, ura4-D18, ade6-M216. The complete ORF coding for Hrad1 was cloned into the SmaI site of the S. pombe expression vector pREP3X (28) using standard techniques. Transformation of S. pombe was carried out by electroporation (29).

Bacterial Expression and Purification of HisHrad1A and HisHrad1B-- The following primers were used to amplify the complete coding regions of Hrad1A and Hrad1B: OAP105 (5'-GCACAGATCTCCCCTTCTGACCCAACAGATC-3') and OAP110 (5'-GCACAGATCTTGTTACCAAGGTTATGGTTAC-3') for the 5' end of Hrad1A and Hrad1B, respectively, each in combination with the 3' PCR primer OAP033 (5'-GCCGAATTCTCAAGACTCAGATTCAGGAA-3'). The PCR product was digested with BglII and EcoRI and cloned into the corresponding sites of pRSETB (Invitrogen). This creates a fusion protein with the following amino acid sequence N-terminal to the second codon of Hrad1A or Hrad1B: MRGSHHHHHHGMASMTGGQQMGRDLYDDDDKDPSS. The insert sequence of both constructs was verified.

Exonuclease Assays-- The DNA used as exonuclease substrate was plasmid pBSII (KS⁺) (Stratagene), digested to completion with Sau3A. The DNA was either labeled at the 3' termini by incorporation of [-³²P]dCTP (~3000 Ci/mmol) using the Klenow fragment of E. coli DNA polymerase or at the 5' termini with [-³²P]ATP (~3000 Ci/mmol) using T4 polynucleotide kinase, prior to filling in 3' recessed termini with unlabeled dNTPs.

Fluorescence in Situ Hybridization Studies (FISH)-- Chromosomal localization was carried out by SeeDNA Inc. (Toronto, Ontario, Canada). Lymphocytes isolated from human blood were cultured in -minimal essential medium supplemented with 10% fetal calf serum and phytohaemagglutinin at 37 °C for 68-72 h. The lymphocyte cultures were treated with bromodeoxyuridine (0.18 mg/ml) to synchronize the cell population. The synchronized cells were washed three times with serum-free medium to release the block and recultured at 37 °C for 6 h in -minimal essential medium with thymidine (2.5 µg/ml). Cells were harvested, and slides were prepared by using standard procedures including hypotonic treatment, fixation, and air drying.

	RESULTS

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Identification of Human and Mouse Homologues of S. pombe rad1⁺-- A human EST cDNA clone (accession number AA029300) was identified in the EMBL DNA data base using the TBLASTN homology searching program (26) with the S. pombe rad1⁺-derived amino acid sequence as the query. This clone was obtained from the I.M.A.G.E. Consortium (EST 470124), and DNA sequence analysis of the 1.2-kb insert (Figs. 1 and 2A) revealed an ORF that was highly similar to S. pombe Rad1. The predicted amino acid sequence of this ORF, however, was significantly shorter than the S. pombe Rad1 protein. To ensure that we had identified the complete sequence of the putative Hrad1 cDNA, we carried out 3' and 5' RACE-PCR. The 3' RACE clones that we obtained were all identical to the original EST cDNA sequence. The 5' RACE clones that we obtained separated into two classes. The first was identical to our original cDNA clone, and the second contained a 119-nucleotide insertion presumably originating from an alternatively spliced mRNA. This additional sequence changed the reading frame immediately upstream of the initiation codon in our original cDNA sequence and resulted in a second and longer ORF (Figs. 1 and 2A). We have named the long and short ORFs Hrad1A and Hrad1B, respectively, and their sequences have been deposited into GenBank^TM under accession codes AJ004974 and AJ004975, respectively). Hrad1A has 60% identity to the S. pombe rad1⁺ sequence at the DNA level and 49% identity and 72% similarity at the amino acid level (Fig. 2B). The Hrad1A cDNA encodes a protein with a predicted molecular mass of approximately 31.5 kDa, and the Hrad1B protein has a predicted molecular mass of approximately 19.5 kDa.

View larger version (52K): [in this window] [in a new window]   Fig. 1.   Nucleotide sequence and predicted amino acid sequence of human rad1. The sequence presumed to be alternatively spliced is underlined, and the two presumptive initiation codons are shown in bold type. The sequences of Hrad1A and Hrad1B have been deposited into GenBankTM under accession codes AJ004974 and AJ004975, respectively.

View larger version (59K): [in this window] [in a new window]   Fig. 2.   Presumptive alternative splice and domain structure of Hrad1. A, schematic diagram of the proposed alternatively spliced forms of human rad1. B, amino acid sequence alignment between human Rad1, S. pombe Rad1, S. cerevisiae RAD17, and U. maydis REC1 performed using the CLUSTALW alignment program. Residues conserved between at least three species are highlighted in gray, those conserved in all four are highlighted in black.

Northern Blot Analysis of Human and Mouse rad1-- The transcript profiles of human and mouse rad1 were examined by probing several multiple tissue Northern blots (CLONTECH), a cancer cell line Northern blot (CLONTECH), and a mouse developmental Northern blot (CLONTECH). Three transcripts of approximately 5, 3, and 1.3 kb were identified for Hrad1, and all were elevated in the cancer cell lines (Fig. 3A). All three transcripts were present in differentiated tissues with the highest levels in testis, skeletal muscle, placenta, and heart. There was a specific increase in the shortest transcript in testis. The alternative splice we identified represents a difference of 119 nucleotides, which is too small to account by itself for the differences between the three transcripts.

View larger version (63K): [in this window] [in a new window]   Fig. 3.   Northern blot analysis of Hrad1 and Mrad1. A, the multiple tissue Northern blot and cancer cell line Northern filters (CLONTECH) were hybridized with the Hrad1B-derived complete ORF. RNA size markers are indicated. The blots were then rehybridized with a human -actin cDNA probe (CLONTECH) to verify the loading of comparable amounts of RNA across lanes as well as the lack of degradation of the samples. B, the center blot was then hybridized with an oligonucleotide specific for the alternatively spliced region of Hrad1 to discriminate between the Hrad1A and Hrad1B transcripts. C, a mouse multiple tissue Northern blot and a developmental Northern blot (CLONTECH) were hybridized with a mouse rad1 cDNA probe. The blots were then rehybridized with a human -actin cDNA probe (CLONTECH). Exposure times were overnight for the Hrad1 probe and 30 min for the -actin probe.

Complementation of S. pombe rad1-- Complementation has often been used to demonstrate biological activity for mammalian homologues of yeast proteins (34, 35). We examined whether Hrad1 could complement the UV irradiation (DNA damage-dependent checkpoint) or hydroxyurea (DNA replication-dependent checkpoint) sensitivity phenotypes of a S. pombe rad1::ura4⁺ strain. Hrad1A and Hrad1B were cloned into the S. pombe expression vector pREP3x (28) and transformed into wild-type and rad1::ura4⁺ cells. Transformants were exposed to varying doses of UV or transiently exposed to 10 mM hydroxyurea as described previously (16). We observed no complementation of the UV or hydroxyurea sensitivity phenotypes (data not shown).

Hrad1 Transcription Is Not DNA Damage-inducible-- Many genes involved in the response to DNA damage are induced by exposure to DNA damaging agents. To examine whether this was the case for Hrad1 we exposed human HaCaT cells to 30 J/m² of UV radiation, a dose sufficient to cause DNA damage and invoke a cell cycle arrest (36). We prepared RNA at various time points up to 8 h after exposure and then determined the effect upon Hrad1 transcript level using an RNase protection assay. The level of the Hrad1 transcript remained constant throughout the time course (data not shown). Consequently, we conclude that Hrad1 transcription is not increased in response to UV-induced DNA damage.

Affinity Purified HisHrad1A Exhibits Exonuclease Activity-- The sequence of Hrad1 is highly similar to that of REC1 from U. maydis, and purified hexahistidine-tagged Rec1 has been shown to have exonuclease activity (21). To test whether Hrad1 shared this biochemical activity, both Hrad1A and Hrad1B were expressed in E. coli as N-terminal hexahistidine-tagged fusion proteins using the pRSETB inducible expression vector (Invitrogen). The proteins designated HisHrad1A and HisHrad1B were purified by immobilized metal affinity chromatography (Qiagen). Bacterially expressed HisHrad1A was found predominantly in the insoluble fraction; however, approximately 20% of the protein as judged by Coomassie staining of SDS-polyacrylamide gels was present in the soluble fraction, allowing purification under nondenaturing conditions. HisHrad1B was found exclusively in the insoluble fraction and was isolated from inclusion bodies under denaturing conditions and refolded by rapid dilution. HisHrad1A and HisHrad1B have predicted molecular masses of approximately 35.5 and 23.5 kDa, respectively. The electrophoretic mobility of the purified proteins on SDS-polyacrylamide gels was in agreement with these predictions (Fig. 4).

View larger version (43K): [in this window] [in a new window]   Fig. 4.   Purification of HisHrad1A and HisHrad1B. Aliquots of the various steps in the purification of HisHrad1A were analyzed by SDS-polyacrylamide gel electrophoresis and Coomassie Blue staining. Size markers are indicated in kDa on the left side. Lane 1, lysate of uninduced cells (equivalent to 20 µl of culture) containing HisHrad1A expression plasmid. Lane 2, lysate of induced cells (equivalent to 20 µl of culture) containing HisHrad1A expression plasmid. Lane 3, soluble fraction from induced cells (equivalent to 300 µl of culture) containing HisHrad1A expression plasmid. Lane 4, insoluble fraction from induced cells (equivalent to 300 µl of culture) containing HisHrad1A expression plasmid. Lane 5, affinity purified HisHrad1A (2 µg). Lane 6, affinity purified HisHrad1B (2 µg).

View larger version (17K): [in this window] [in a new window]   Fig. 5.   Exonucleolytic activity of affinity purified HisHrad1A. The affinity purified HisHrad1A and HisHrad1B were examined for exonuclease activity. The results are the means of four experiments. A, 3 µg of protein was incubated under standard reaction conditions with 3' 32P end-labeled double-stranded DNA for the times indicated. HisHrad1B exhibited no 3'  5' exonuclease activity. B, 3 µg of protein was incubated under standard reaction conditions for 30 min with 3' and 5' 32P end-labeled double- and single-stranded DNA.

View this table: [in this window] [in a new window]   Table II Comparison of nuclease activities with respect to substrate The data for REC1, exonuclease I,  exonuclease, and exonuclease III are taken from Thelen et al. (21). All Hrad1A reactions were carried out as described under "Experimental Procedures" for 30 min at 37 °C. Reaction conditions for the other (cited) enzymes were similar but not identical, so that exact numerical comparisons are not warranted.

Chromosomal Localization of Hrad1-- The chromosomal position of Hrad1as determined to establish whether a loss of heterozygosity associated with Hrad1 might be linked with any known disease. The 1.2-kb cDNA corresponding to Hrad1B (EST 470124) was used as a probe for FISH analysis. Under the conditions used, the hybridization efficiency was approximately 69% for the probe (among 100 checked mitotic figures, 69 showed signals on one pair of chromosomes). The DAPI banding pattern was used to establish that Hrad1 localizes to the short arm of chromosome 5. No additional locus was picked up by FISH detection under the conditions used. The detailed position was further determined based upon the analysis of 10 photographs leading to the conclusion that Hrad1 is located on human chromosome 5p13.3-13.2 (Fig. 6). Loss of heterozygosity of this region of chromosome 5 has been linked to a variety of human neoplasias including lung cancer (37, 38).

View larger version (30K): [in this window] [in a new window]   Fig. 6.   Chromosomal localization of the Hrad1 gene determined by FISH analysis. A, photograph of metaphase chromosome spread labeled with fluorescent Hrad1 cDNA probe. B, photograph of the same mitotic figure stained with DAPI to identify chromosomes and reveal chromosome band patterns. Comparison with panel A shows the FISH signal is located on chromosome 5. C, idiogram of chromosome 5. The signal was further localized to position p13.2-p13.3 by determination of the distribution of signals from 10 independent photographs.

	DISCUSSION

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In S. pombe, cell cycle checkpoint arrest in response to DNA damage or inhibition of replication is dependent on multiple proteins. Six gene products have been identified that act early in the process of checkpoint control, and to date human homologues of two of these proteins have been identified, Hrad9 (9) and ATR (7, 8). Based upon sequence homology and biochemical activity, we have identified a third component, Hrad1, a human homologue of he S. pombe rad1⁺ cell cycle checkpoint gene. Hrad1 is highly similar to S. pombe rad1⁺, S. cerevisiae RAD17, and the REC1 gene of U. maydis. Mutations in these genes lead to similar cell cycle checkpoint defects and other phenotypes associated with alterations in DNA repair and recombination (16, 19, 20).

We have shown that Hrad1 is subject to alternative splicing giving rise to two ORFs, a long form, designated Hrad1A, and an N-terminal truncation, designated Hrad1B. The Hrad1 gene has at least three transcripts of 5, 3, and 1.3 kb present in all tissues that we examined and that are increased in all the cancer cell lines examined, suggesting either that transcription of Hrad1 is proliferation-dependent or that it is increased in response to the genomic instability of cancer cell lines. In testis we observed an increase specifically in the 1.3-kb transcript. We have demonstrated that this transcript does not correspond to Hrad1A, suggesting that it corresponds to Hrad1B or an as yet unidentified third form of Hrad1. Several yeast cell cycle checkpoint genes play important roles in meiosis (39) and recently ATM and ATR (the human homologues of S. pombe rad3) were shown to be highly expressed in testis where they interact with meiotic chromosomes. This suggests a direct role for these proteins in recognizing and responding to DNA strand interruptions that occur during meiotic recombination (40). Hrad1 could form part of this recognition complex in association with ATM or ATR.

When expressed in an S. pombe rad1::ura4⁺ strain, both Hrad1A and Hrad1B failed to complement the phenotypes associated with loss of checkpoint function. However, this should not be taken as evidence against functional homology because failure to complement has been shown for other checkpoint genes such as ATR and Hchk1 (7, 12).²

The Hrad1 amino acid sequence is significantly similar to the U. maydis REC1 protein, indicating that Hrad1 may have a 3'  5' exonuclease activity. We have confirmed the predicted 3'  5' exonuclease activity for Hrad1A by demonstrating that Hrad1A expressed as a hexahistidine fusion protein and purified from E. coli is an exonuclease with a 6-fold preference for 3' termini. The biochemical properties of HisHrad1A were very similar to REC1 (21). The truncated Hrad1B, expressed as a hexahistidine fusion protein, lacks detectable exonuclease activity. This may reflect the in vivo biological activity or may be a consequence of the purification regimes. To fully address this question, purification of endogenous Hrad1B from mammalian cells or from a heterologous expression system that yields native protein will be required.

An analysis of C-terminal truncations of the Rec1 protein has established that the C-terminal portion of the Rec1 protein is not essential for exonuclease activity but is crucial for G₂-M checkpoint function (24, 25). The checkpoint-compromised U. maydis rec1-1 mutant results from a C-terminal truncation lacking 122 amino acids (24), including a block of sequence containing four periodically spaced leucines that is conserved in Hrad1 and S. pombe Rad1 but not in S. cerevisiae RAD17 (24). This may represent a domain important in interactions with other checkpoint components. In addition, the rec1-5 mutant that lacks exonuclease activity has a 100-fold higher rate of spontaneous mutation, indicating that the exonuclease function may be required for mismatch repair (24). Thus, the Rec1 protein appears bifunctional, having an N-terminal exonuclease domain and a C-terminal checkpoint domain. This type of separation of enzymatic and checkpoint domains has also been described for S. cerevisiae DNA polymerase  (41). The bifunctional nature is likely to be conserved in Hrad1A based upon our biochemical studies and the protein sequence similarity between Hrad1A and the U. maydis Rec1 protein. However, in humans we have identified an additional component, Hrad1B, which corresponds to the C-terminal region of Rec1 implicated solely in checkpoint control. We cannot discount the possibility that the alternatively spliced mRNA coding for Hrad1B is not translated. However, if the Hrad1B mRNA leads to the production of a protein, then what role might it play in DNA metabolism or checkpoint control? In fission yeast, rad1 is required for both the DNA damage- and DNA replication-dependent checkpoints, whereas in S. cerevisiae RAD17 is only required for the DNA damage-dependent checkpoint. It has been demonstrated that RAD17 functions in conjunction with RAD24 and MEC1 to activate DNA degradation (23), leading to the suggestion that there is a requirement to process single- or double-stranded breaks such that single-stranded DNA is exposed to activate the checkpoint (22, 23). If we consider Hrad1 as part of a complex that acts as a surveillance mechanism monitoring the genome and communicating the presence of DNA damage or unreplicated DNA to the cell cycle machinery, then it is conceivable that in higher eukaryotes two types of surveillance complexes exist separating the DNA damage- and DNA replication-dependent checkpoint pathways. Hrad1A could be specifically required for the DNA damage-dependent checkpoint, able to monitor the genome via the C-terminal checkpoint domain and process single- and double-stranded breaks via the N-terminal exonuclease domain. A link between the mismatch repair system and the DNA damage-dependent checkpoint has been demonstrated (42). Hrad1B may have a function in the DNA replication-dependent checkpoint that might require only the monitoring activity and not the nuclease function to identify replication forks. Hrad1B also contains the sequence shown to be essential for checkpoint function in a mutational analysis of S. pombe Rad1 (43). The increase in transcription that we observed in testis could also signify a role for Hrad1B in meiotic recombination in conjunction with ATR and ATM.

The loss of checkpoint function has been shown to lead to genomic instability even in the absence of exogenous DNA damage (44). In humans, the p53 gene and the ATM gene are not only required for the G₁-S phase checkpoint (45, 46) but also act as tumor suppressors (47-50). It is likely that other checkpoint genes will act as tumor suppressors. The Hrad1 gene is located on chromosome 5p13.2-13.3. Loss of heterozygosity of this region of chromosome 5 has been associated with a tumor suppressor function most notably in human lung cancer (37, 38). The CDK2/cyclin A-associated protein p45 (SKP2) has been mapped to 5p13 (51), but Hrad1 should also be considered as a candidate tumor suppressor gene in this region.

The precise roles that Hrad1A and Hrad1B play in cell cycle control, DNA repair, and tumor suppression remain to be determined. This study represents a starting point to begin to unravel the potential multiple roles for the Hrad1 gene product.