Trinucleotide instability: a repeating theme in human inherited disorders.

In recent years, a completely new mechanism of mutation has emerged in a number of disorders that display perplexing and paradoxical features of genetic inheritance. This mechanism involves the expansion and intergenerational instability of stretches of consecutive identical nucleotide triplets that also exist as shorter stable segments on normal chromosomes. The unstable nature of the trinucleotide segments has solved many of the genealogic puzzles in these disorders and has provided a new tool for predictive testing. Treatments for the disorders await a better understanding of the different pathogenic processes that are triggered by various expanded repeats. The existence of numerous other disorders with peculiarities of genetic inheritance suggests that this mutational mechanism may be a major cause of human inherited disease.