The fragile X-related disorders (FXDs) are members of the group of diseases 
known as the repeat expansion diseases. The FXDs result from expansion of an 
unstable CGG/CCG repeat tract in the 5' UTR of the FMR1 gene. Contractions are 
also seen, albeit at lower frequency. We have previously shown that ERCC6/CSB 
plays an auxiliary role in promoting germ line and somatic expansions in a mouse 
model of the FXDs. However, work in model systems of other repeat expansion 
diseases has suggested that CSB may protect against expansions by promoting 
contractions. Since FXD mice normally have such a high expansion frequency, it 
is possible that such a protective effect would have been masked. We thus 
examined the effect of the loss of CSB in an Msh2(+/-) background where the germ 
line expansion frequency is reduced and in an Msh2(-/-) background where 
expansions do not occur, but contractions do. Our data show that in addition to 
promoting repeat expansion, CSB does in fact protect the genome from germ line 
expansions in the FXD mouse model. However, it likely does so not by promoting 
contractions but by promoting an error-free process that preserves the parental 
allele.