Spinocerebellar ataxia type 1 (SCA1) is one an intriguing set of nine neurodegenerative diseases caused by the expansion of a unstable trinucleotide CAG repeat where the repeat is located within the coding of the affected gene, i.e. the polyglutamine (polyQ) diseases. A gain-of-function mechanism for toxicity in SCA1, like the other polyQ diseases, is thought to have a major role in pathogenesis. Yet, the specific nature of this gain-of-function is a matter of considerable discussion. An issue concerns whether toxicity stems from the native or normal function of the affected protein versus a novel function induced by polyQ expansion. For SCA1 considerable evidence is accumulating that pathology is mediated by a polyQ-induced exaggeration of a native function of the host protein Ataxin-1 (ATXN1) and that phosphorylation of S776 regulates its interaction with other cellular protein and thereby function. In addition, this posttranslational modification modulates toxicity of ATXN1 with an expanded polyglutamine.