Triplet repeats and human disease

In recent years a new mechanism of genetic disease has been discovered in which triplet DNA sequences (trinucleotides) expand either to interrupt or to compromise a gene. So far, ten genetic loci have been identified in which this event takes place. The presence of these expansions, sometimes containing thousands of repeated trinucleotides, provides a clue as to how processing of DNA and RNA may sometimes go awry. How this expansion in a single allele may exert a dominant effect and how the extent of the expansion increases the severity of the disease remain a mystery.