As inferred from mouse homologs, after binding PIWIL1 (HIWI in human, Miwi in mouse) the 3' end of the pre-piRNA is trimmed by an unknown nuclease. The final size of the piRNA appears to be determined by the particular PIWI protein with which it is associated. PIWIL1 and TDRD6 are located in the chromatoid body. Both TDRD6 and TDRKH are associated with PIWIL1 in adult testes but only TDRKH is present in embryonic prospermatogonia. TDRKH is required for spermatogenesis and appears to participate in trimming of the 3' end of pre-piRNAs.