C9orf72 and frontotemporal dementia: The C9orf72 hexanucleotide repeat expansion (HRE) is the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) in populations of European descent (Renton et al., 2011; Majounie et al., 2012) and especially common in Finland (Laaksovirta et al., 2022).