C9orf72 and frontotemporal dementia: These findings are of clinical importance in terms of potential causes of autosomal dominant PPA: GRN mutations are the most common cause of this but mutations in other FTD genes such as C9orf72 and MAPT only very rarely cause PPA (Moore et al., 2020), and so one should include a search for a TBK1 mutation in those who are GRN-negative.