ATXN2 and frontotemporal dementia: In contrast, ATXN2 intermediate-length repeat expansions in the range of 27 to 33 units act within oligo- or polygenic backgrounds to increase the disease risk or progression of amyotrophic lateral sclerosis (ALS)9–11, frontotemporal dementia (FTD)12,13, progressive supranuclear palsy (PSP)14, and idiopathic levodopa-responsive Parkinson’s disease15.