C9orf72 and frontotemporal dementia: However, the commonest cause of both familial ALS and FTD is a hexanucleotide GGGGCC expansion in C9orf72, which may be associated with relatively pure forms of ALS and FTD, or their combination (ALS‐FTD) within the same pedigree.7 In the latter group, behavioral change is typically one of the first clinical features, and may include frank psychosis,8 such as might be misdiagnosed as schizophrenia or bipolar disorder.