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Simplified representation of the currently proposed model of piRNA biogenesis. Two steps of piRNA biogenesis: (1) primary amplification cycle in nucleus and (2) secondary amplification cycle described as “ping-pong cycle” in cytoplasm. Abbreviations: Pol II: polymerase II; Zuc: endonuclease Zucchini; Aub: Aubergine protein; Ago 3: Argonauta 3 protein.
