The most common methods for diagnosing prion diseases rely on differentiating between PrPC and PrPSc in post mortem samples by their distinct resistances to proteinase K (PK): under defined conditions, PrPC is completely degraded and PrPSc is cleaved at the N-terminus leaving a protease resistant core of 27 to 30 kDa (Figure 1). The gene discussed is PRNP; the disease is prion disease.