The S18Y polymorphism in the UCHL1 gene is a genetic modifier in Huntington's disease.

An expanded polyglutamine stretch in the huntingtin protein has been identified as the pathogenetic cause of Huntington's disease (HD). Although the length of the expanded polyglutamine repeat is inversely correlated with the age-at-onset, additional genetic factors are thought to modify the variance in the disease onset. As linkage analysis suggested a modifier locus on chromosome 4p, we investigated the functional relevance of S18Y polymorphism of the ubiquitin carboxy-terminal hydrolase L1 in 946 Caucasian HD patients. In this group, the allelic variation on locus S18Y is responsible for 1.1% of the variance in the HD age-at-onset, and the rare Y allele is associated with younger-aged cases.