C9orf72 and frontotemporal dementia: These each account for about 5–10% of all FTD cases although there is geographical variability, e.g. in a US series, C9orf72 expansions were the most common cause (7% of all FTD, compared to 5% GRN and 4% MAPT) (Dejesus-Hernandez et al., 2011) whereas in a Dutch series, MAPT mutations were the most common cause (10% of all FTD, compared to 9% C9orf72 and 7% GRN) (Simón-Sánchez et al., 2012), and in a UK series, the frequency of mutations in each of the genes was approximately equal (7% C9orf72, 7% GRN, 6% MAPT) (Mahoney et al., 2012).