Clinically, while there is significant overlap, patients typically present with either a behavioural variant FTD or amyotrophic lateral sclerosis as their main phenotype31, and they can progress at various rates; genetically, the expansion length is variable and there are additional genetic modifiers (e.g. TMEM106B and ATXN2) that alter phenotype32–34; and pathologically, most cases have either type A or type B TDP-43 pathology31. Here, ATXN2 is linked to frontotemporal dementia.