Fig. 1.

piRNA biosynthesis. Two major pathways can generate piRNA. In primary piRNA biogenesis the PIWI protein, here MILI, cuts a precursor RNA into piRNA. In secondary piRNA biogenesis piRNAs in a sense orientation to active full-length copies of retrotransposon LINE1 (blue) bound to MILI target cleavage of transcripts containing fragments of LINE1 (3′-terminal fragment depicted as “NE1”) transcribed by antisense-oriented promotion. These new antisense piRNA can be incorporated in complex with MIWI2 to cleave the sense-oriented transcripts of LINE1 to generate more sense piPRNA to repeat the cycle.