PIWIL4 and cancer: The human PIWI subfamily of Argonaute proteins comprise 4 members, PIWIL1/HIWI, PIWIL2/HILI, PIWIL3 and PIWIL4/HIWI2 [11], all found in testis, although recently a number of reports have identified elevated expression of HIWI and HILI in a variety of human cancers [12].