Both genes (indicated in red) contain 67 short exons that align in 1:1 correspondence to one another; these exons are connected by curved lines. The human gene is 7.5 times larger than the Fugu gene (180,000 versus 24,000 nucleotide pairs). The size difference is entirely due to larger introns in the human gene. The larger size of the human introns is due in part to the presence of retrotransposons, whose positons are represented by green vertical lines; the Fugu introns lack retrotransposons. In humans, mutation of the huntingtin gene causes Huntington's disease, an inherited neurodegenerative disorder (see p. 362). (Adapted from S. Baxendale et al., Nat. Genet. 10:67–76, 1995.)