The Piwi-interacting RNA (piRNA) biogenesis pathway in mouse testis. The piRNA biogenesis pathway in mouse can be categorized into three modes. MILI is expressed in both prenatal and adult testis. MIWI2 is expressed in prenatal testis and its expression decreased after birth and is not detectable in adult testis. MIWI is expressed in adult testis. (A) When MILI and MIWI2 are coexpressed in prenatal testis, the primary piRNA transcript is processed for loading into MILI. The MILI-piRISC can form homotypic ping-pong amplification loop. MIWI2-associated piRNAs are processed from anti-sense transcripts with the aid of MILI-piRNA-induced silencing complex (piRISC). Therefore, the production of MIWI2-associated piRNA depends on mature MILI-piRISC. (B) When only MILI protein is expressed in testis, MILI process sense and anti-sense piRNA precursor transcripts. (C) When MILI and MIWI are coexpressed in adult testis, both Piwi proteins process the sense and anti-sense piRNA precursor transcript.