Elovl1 is a ubiquitously expressed gene the product of which belongs to a highly conserved family of microsomal enzymes which are involved in the formation of very long chain fatty acids and sphingolipids from yeast to man. In order to elucidate the structure and regulation of the Elovl gene we have isolated a lambda phage genomic DNA clone containing the entire mouse gene, and found that Elovl1 consists of eight exons which are dispersed over 5.4 kb of genomic sequence. Interestingly, sequencing of the lambda clone to completion revealed that the insert contained a segment of the cell cycle gene p55Cdc directed in the opposite orientation. The genes are very tightly linked, so that the 3' ends of their long mRNA species are complementary over a short stretch of nucleotides. Although both Elovl1 and p55Cdc are highly conserved genes, a BLAST search implies that the tail-to-tail arrangement has evolved in vertebrates. Despite the non-similar expression pattern in different tissues, mRNA analysis of the two genes disclosed simultaneous transcription during a proliferation-differentiation transition state which suggests that the two genes may be regulated through a common bi-directional transcription mechanism under specific conditions.