Any heritable epigenetic process that modulates the frequency, rate or extent of protein function by self-perpetuating conformational conversions of normal proteins in healthy cells. This is distinct from, though mechanistically analogous to, disease states associated with prion propagation and amyloidogenesis. A single protein, if it carries a glutamine/asparagine-rich ('prion') domain, can sometimes stably exist in at least two distinct physical states, each associated with a different phenotype; propagation of one of these traits is achieved by a self-perpetuating change in the protein from one form to the other, mediated by conformational changes in the glutamine/asparagine-rich domain. Prion domains are both modular and transferable to other proteins, on which they can confer a heritable epigenetic alteration of function; existing bioinformatics data indicate that they are rare in non-eukarya, but common in eukarya.