PRNP and prion disease: However, since the function of the PrP protein remains evasive, and lack of function mutants do not seem to have major complications but rather subtle effects at advanced ages [38], we wondered if the driver of evolution for this protein could had been their susceptibility to prion diseases rather than the function of the physiological isoform of the protein, as the only known dramatic effect of alterations in the PRNP gene are related to prion disease susceptibility [39].