 Most cases of SCA6 are a result of CAG repeat expansion beyond the normal range, i.e., more than 19 repeats, in the Cav2.1 calcium channel encoding gene CACNA1A.This gene has two splice forms, "Q-type" and "P-type", and the polyglutamine coding CAG expansion occurs in the P-type splice form.This form is expressed heavily in the cerebellum where it is localized in Purkinje cells.