John P. Jakupciak and Robert D. Wells
Genetics Program, Texas A&M University, Institute of Biosciences and
Technology, Center for Genome Research, 2121 W. Holcombe Blvd. Houston,
Texas 77030

  The expansion of triplet repeat sequences is an initial step in the
disease etiology of a number of hereditary neurological disorders in
humans.  Diseases such as myotonic dystrophy, Huntington's, several
spinocerebellar ataxias, fragile X syndrome, and Freidreich's ataxia are
caused by the expansions of CTG CAG, CGG CCG, or GAA TTC repeats.  The
mechanisms of the expansion process have been investigated intensely in E.
coli, yeast, transgenic mice, mammalian cell culture, and in  human
clinical cases.  Whereas studies from 1994-1999 have implicated DNA
rep[lication and repair at the paused DNA synthesis sites due to the
unusual conformations of the triplet repeat sequences, recent work has
shown that homologous recombination (gene conversion) is a powerful
mechanism for generating massive expansions, in addition to, or in concert
with, replication and repair.