C9orf72 and frontotemporal dementia: Indirect estimates calculated from the incidence of ALS in first-degree relatives of C9orf72 HRE carriers or from the population incidence of ALS-FTD and population prevalence of C9orf72 HRE have suggested a penetrance of 24%–33%,14,15 but are reliant on the accuracy of these underlying statistics and do not include other potential disease phenotypes.