The expansion of a polymorphic CAG repeat in the HD gene encoding huntingtin has 
been identified as the major cause of Huntington's disease (HD) and determines 
42-73% of the variance in the age-at-onset of the disease. Polymorphisms in 
huntingtin interacting or associated genes are thought to modify the course of 
the disease. To identify genetic modifiers influencing the age at disease onset, 
we searched for polymorphic markers in the GRIK2, TBP, BDNF, HIP1 and ZDHHC17 
genes and analysed seven of them by association studies in 980 independent 
European HD patients. Screening for unknown sequence variations we found besides 
several silent variations three polymorphisms in the ZDHHC17 gene. These and 
polymorphisms in the GRIK2, TBP and BDNF genes were analysed with respect to 
their association with the HD age-at-onset. Although some of the factors have 
been defined as genetic modifier factors in previous studies, none of the genes 
encoding GRIK2, TBP, BDNF and ZDHHC17 could be identified as a genetic modifier 
for HD.