PRNP and Creutzfeldt Jacob disease: In order to determine whether differences in CJD phenotype correlated with differences in the relative amounts of PrPSc-M129 and PrPSc-V129, we extracted protease-resistant PrPSc from brain tissue of multiple sCJD and iCJD patients heterozygous for M and V at codon 129 in PRNP. Tandem mass spectrometry was then used to differentiate peptides containing M at residue 129 from those containing V. Our results show that the relative abundance of PrPSc-M129 and PrPSc-V129 was highly variable between individual sCJD cases.