PRNP and Alzheimer disease: Genetically, mutations of PRNP were found in clinically diagnosed AD and FTD patients, and the homozygosity of codon 129 (M/M alleles) was associated with higher risk for CJD, whereas the heterozygosity of codon 129 (M/V alleles) is a possible risk factor for AD and may modify the age at onset of FTD in some Caucasian populations20, 21.