Objective: The aim of this study was to establish the frequency of ATXN2 polyglutamine (polyQ) expansion in large cohorts of patients with amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and progressive supranuclear palsy (PSP), and to evaluate whether ATXN2 could act as a modifier gene in patients carrying the C9orf72 expansion.Methods: We screened a large cohort of French patients (1,144 ALS, 203 FTD, 168 FTD-ALS, and 109 PSP) for ATXN2 CAG repeat length. We included in our cohort 322 carriers of the C9orf72 expansion (202 ALS, 63 FTD, and 57 FTD-ALS).Results: We found a significant association with intermediate repeat size ($29 CAG) in patients with ALS (both familial and sporadic) and, for the first time, in patients with familial FTD-ALS. Of interest, we found the co-occurrence of pathogenic C9orf72 expansion in 23.2% of ATXN2 intermediate-repeat carriers, all in the FTD-ALS and familial ALS subgroups. In the cohort of C9orf72 carriers, 3.1% of patients also carried an intermediate ATXN2 repeat length. ATXN2 repeat lengths in patients with PSP and FTD were found to be similar to the controls.Conclusions: ATXN2 intermediary repeat length is a strong risk factor for ALS and FTD-ALS.Furthermore, we propose that ATXN2 polyQ expansions could act as a strong modifier of the FTD phenotype in the presence of a C9orf72 repeat expansion, leading to the development of clinical signs featuring both FTD and ALS. Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are considered part of a spectrum of neurologic diseases because they share pathologic hallmarks, consisting of TAR DNA-binding protein 43 (TDP-43) pathology, and a similar genetic background, including C9orf72 expansion. On the other hand, FTD shares a prominent frontal cognitive syndrome and parkinsonism signs with progressive supranuclear palsy (PSP). Spinocerebellar ataxia type 2 (SCA2), characterized by cerebellar ataxia, parkinsonism, and mild dementia, is caused by an expanded CAG trinucleotide repeat encoding for a polyglutamine (polyQ) tract in ataxin-2 protein (ATXN2; OMIM: *601517). A link between SCA2 and ALS was established in 2010, when ATXN2 intermediate-length expansions were found to be significantly associated with ALS and it was shown that ATXN2 protein modifies TDP-43 toxicity.1 In this study, we analyzed the size of ATXN2 repeats in a large French cohort of 1,624 patients with ALS, FTD, FTD-ALS, and PSP to evaluate the exact contribution of the ATXN2 repeat size to