We found that exposure to mutant ATXN1 in young, developing mice caused ataxia and disrupted CF, but not ML, architecture from as early as 4 weeks of age; in contrast mice exposed to ATXN1 from 6 weeks of age when cerebellar development was over, exhibited normal behavior and cerebellar architecture remained largely intact for up to 6 months. The gene discussed is ATXN1; the disease is cystic fibrosis.