Mutations in any of 4 genes are causative of early onset dementias due to either AD (presenilin 1 [PSEN1], presenilin 2 [PSEN2], and amyloid precursor protein [APP]) or prion disease (prion protein gene, PRNP) (Collinge, 2001; Hardy and Orr, 2006). Here, PRNP is linked to Alzheimer disease.