Missense mutations in the gene encoding the IP3R (ITPR1) cause Spinocerebellar Ataxia type 15 (66), while the polyglutamine repeat expansion proteins ataxin-2 and huntingtin bind to the IP3R C terminus and increase its sensitivity to IP3 (67, 68). This evidence concerns the gene HTT and spinocerebellar ataxia type 15/16.