 Human Piwi proteins all contain two RNA binding domains, PAZ and Piwi.The four PIWI-like proteins have a spacious binding site within the PAZ domain which allows them to bind the bulky 2’-OCH3 at the 3’ end of piwi-interacting RNA.One of the major human homologues, whose upregulation is implicated in the formation of tumours such as seminomas, is called "hiwi ("for human piwi")."