The genetical basis of SCA1 is an expansion of a polymorphic CAG trinucleotide repeat within the coding region of the ataxin-1 gene on chromosome 6p [1]. 

There is a wide heterogeneity of the mean age at onset in SCA1 patients [2]. Observed inter-familial dif­ferences in the age at onset may be due to the differences in the length of the CAG trinucleotide repeats. The number of CAG repeats correlates inversely with the age at onset and directly with the severity of the disease [3]. However, besides genetic, the environmental factors, as well as the interaction between genetic and environmental factors, may also influence clinical expression and fea­tures of SCA1.

Herein, we report two large, unrelated families with SCA1 from the same village in the southeastern, moun­tain region of Serbia, which significantly differ in the age at onset, despite the lack of difference in the mean number of CAG trinucleotides in the ataxin-1 gene.