MAPT and frontotemporal dementia: Recently, Geschwind and collaborators have shown that a rare variant in the MAPT gene, where mutations are known to be causative for FTD, can act as a risk factor for FTD and Alzheimer's disease.13 Similarly, we here suggest that a rare variant in PFN1, a gene in which mutations are known to be causative for ALS, can act as a risk factor for disease.