PRNP stands for “prion protein,” and numerous mutations in this gene are associated with the familial (inherited) form of Creutzfeldt-Jakob Disease (CJD). The possibility of familial transmission of CJD is the reason that U.S. blood donors are asked if any blood relatives have CJD. This gene has been identified on the short arm of chromosome 20, and can be tested by DNA sequencing to identify characteristic mutations in a patient at risk for familial CJD. According the the FDA, negative PRNP mutation testing allows a potential blood donors with a family history of CJD in a blood relative to resume blood donation.