The -amyloid peptide (A) is the major constituent of the amyloid core of senile plaques found in the brain of patients with Alzheimer’s disease (AD). A is produced by the sequential cleavage of the Amyloid Precursor Protein (APP) by - and -secretases. Cleavage of APP by -secretase also generates the APP Intracellular C-terminal Domain (AICD) peptide, which might be involved in regulation of gene transcription. APP contains three glycine-xxx-glycine (GxxxG) motifs in its juxtamembrane and transmembrane (TM) regions. Such motifs are known to promote dimerization via close apposition of TM sequences. We demonstrate that pairwise replacement of glycines by leucines or isoleucines, but not alanines, in a GxxxG motif led to a drastic reduction of A40 and A42 secretion. -Cleavage of mutant APP was not inhibited, and reduction of A secretion resulted from inhibition of -cleavage. It was anticipated that decreased -cleavage of mutant APP would result from inhibition of its dimerization. Surprisingly, mutations of the GxxxG motif actually enhanced dimerization of the APP C-terminal fragments, possibly via a different TM -helical interface. Increased dimerization of the TM APP C-terminal domain did not affect AICD production. These results clearly demonstrate that both orientation and dimerization of the APP TM domain differently affect A and AICD production