C9orf72 and Alzheimer disease: Previous longitudinal investigations involving patients with a known mutation are limited but show a similar range of global volumetric changes, suggesting MAPT mutations are associated with a mean annual rate of ∼ 1.6%, intermediate between those with GRN mutations, who exhibit the fastest rate of loss at ∼ 3.5% and C9orf72 whose volumetric rates have been reported as the lowest at ∼ 1.4% and more in line with people with AD (Gordon et al., 2010, Rohrer et al., 2010a, Whitwell et al., 2015a, Whitwell et al., 2011).