468 Chapter 14 • Mutation, Repair, and Recombination muscle weakness and atrophy, the expansion of the trinucleotide repeat occurs in the gene that encodes the androgen receptor. Properties common to some trinucleotide-repeat diseases suggest a common mechanism by which the ab-normal phenotypes are produced. First, many of these diseases seem to include neurodegeneration—that is, cell death within the nervous system. Second, in such diseases the trinucleotide repeats fall within the open reading frames of the transcripts of these genes, leading to expansions or contractions of the number of repeats of a single amino acid in the polypeptide (for example, CAG repeats encode a polyglutamine repeat). Thus, it is no accident that these diseases entail expansions of codon-size three-base-pair units. But this explanation cannot hold for all trinucleotide-repeat diseases. After all, in fragile X syndrome, the trinucleotide expansion occurs near the 5 9 end of the FMR-1 mRNA, before the translation start site. Thus, we
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