polypeptide_domain domain BS:00134 BS:00012 A structurally or functionally defined protein region. In proteins with multiple domains, the combination of the domains determines the function of the protein. A region which has been shown to recur throughout evolution. polypeptide_structural_domain sequence polypeptide domain SO:0000417 structural domain SO:0001069 Range. Old definition from before biosapiens: A region of a single polypeptide chain that folds into an independent unit and exhibits biological activity. A polypeptide chain may have multiple domains. polypeptide_structural_region polypeptide_conserved_region