These results indicate that gametogenesis is the primary source of inherited instability in the Hdh knock-in mouse, as it is in man, but that the underlying repeat length-dependent mechanism, which may or may not be related in the two species, operates at higher CAG numbers. Limited CAG variation in the somatic tissues gave way to marked mosaicism in liver and striatum for the longest repeats in older mice. Instead, size changes were modest increases and decreases, with expansions typically emanating from males and contractions from females. No large jumps in CAG length were detected in either male or female transmissions. However, lengthening the repeats in Hdh to 90 and 109 units produced a graded increase in the mutation frequency to >70%, with instability being more evident in female transmissions. A similar pattern was obtained when this repeat was inserted into exon 1 of the mouse Hdh gene. Initial efforts to reproduce these features in a mouse model transgenic for HD exon 1 with 48 CAG repeats revealed only mild intergenerational instability (∼2% of meioses). In a subset of male transmissions, larger size increases occur to produce extreme HD alleles that display somatic instability and cause juvenile onset of the disorder. The CAG repeats in the human Huntington's disease (HD) gene exhibit striking length-dependent intergenerational instability, typically small size increases or decreases of one to a few CAGs, but little variation in somatic tissues.