{{Genotype
|rsid=1799990
|allele1=A
|allele2=A
|magnitude=1
|repute=Bad
|summary=Increased chance of Prion Disease (PrP 129 Met homozygote)
}}
This genotype encodes for a homozygous Methionine at position 129 of [[PRNP]], the Prion Protein gene (PrP-129MM). Unlike the protective M/V heterozgotes, homozygosity at this residue is associated with an increased susceptibility to [[prion disease]] (such as [[Creutzfeldt-Jakob]])

All reported cases of [[vCJD]], (linked to ingestion of 'mad cow disease' contaminated meat) have been 129 Met homozygous {{PMID|22505363}}