UniProtKB description: Acts via the piwi-interacting RNA (piRNA) metabolic process, which mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and governs the methylation and subsequent repression of transposons (PubMed:26808625, PubMed:15817569, PubMed:17346786). Directly binds piRNAs, a class of 24 to 30 nucleotide RNAs that are generated by a Dicer-independent mechanism and are primarily derived from transposons and other repeated sequence elements (PubMed:16882972). In ovarian somatic cells, mediates silencing of transposable elements at the transcriptional level in a mael-dependent manner (PubMed:23159368, PubMed:28472469). Involved in silencing of long terminal repeat (LTR) retrotransposons in male germline (PubMed:15817569). In testis, regulates spermatogenesis together with Tudor-SN (PubMed:26808625). In germ cells, mediates silencing at both transcriptional and post-transcriptional levels and is involved in the maintenance of populations of primary and secondary piRNAs. Piwi-mediated transcriptional silencing is accompanied by the formation of His3 'Lys-9' trimethylated (H3K9me3) euchromatin and heterochromatin (PubMed:23434410, PubMed:23392610). In ovary, associates predominantly with antisense piRNAs that contain uridine at their 5' end. Association with sense piRNAs is also observed but to a lesser extent. Mediates a somatic signaling mechanism required for the maintenance of germline stem cells to produce and maintain a daughter germline stem cell (PubMed:9851978, PubMed:10631171, PubMed:9199372, PubMed:16949822). It is not essential for the further differentiation of the committed daughter cell (PubMed:9851978). Acts cell autonomously to promote germline stem cell division (PubMed:9851978, PubMed:10631171). Its role in stem cell maintenance does not seem to require nuclear localization. Required maternally for the posterior localization of osk and vas and for pole cell formation during oogenesis and early embryogenesis (PubMed:16949822). Together with Hop and Hsp83, mediates canalization, also known as developmental robustness, likely via epigenetic silencing of existing genetic variants and suppression of transposon-induced new genetic variation (PubMed:21186352). Shows RNA cleavage activity, although is not required for any of its known functions (PubMed:9199372, PubMed:16882972, PubMed:23297219). In the ovaries, forms a complex with nxf2, Panx and Nxt1 which acts as effectors of cotranscriptional transposon silencing (PubMed:31368590, PubMed:31384064).