As inferred from homologs in Drosophila and mouse, PLD6 (MitoPLD) located on the cytoplasmic face of the mitochondrial outer membrane makes the first endonucleolytic cleavage of primary piRNA transcripts. The cleavage yields a 5' phosphate and a 3' hydroxyl. Cleavage is believed to precede loading into PIWIL1 (HIWI, MIWI) or PIWIL2 (HILI, MILI). Most mature piRNAs have uracil at the 5' end. This appears to be due to selective binding by PIWI proteins rather than selective cleavage (reviewed in Bortvin 2013).