C9orf72 and frontotemporal dementia: First, TAR DNA-binding protein ∼43 kDa (TDP-43) inclusions are the pathological hallmark of ∼50% of FTD and the vast majority (>98%) of ALS cases.2 Second, hexanucleotide repeat expansions in C9orf723,4 can cause FTD, ALS, or both, and account for ∼5% of simplex and one-third of familial FTD and ALS cases.5,6 Third, clinical overlap between FTD and ALS can occur, even in the absence of a C9orf72 expansion, though estimates of the relative frequency of subsequent feature development vary widely.