C9orf72 and frontotemporal dementia: In a study of genetic FTD, a differential pattern of involvement was seen in the different groups: MAPT mutation carriers showed a 24–27% volumetric difference in the hippocampus proper (formed by the CA subfields), whilst C9orf72 expansion carriers showed most atrophy in the dentate gyrus and CA1/4 (8–11%), and GRN mutation carriers were most affected in the subiculum and presubiculum (10–14%).100