The long-term objective of the current application is to elucidate the biogenesis and function of a novel class of small non-coding RNAs, the piRNAs. RNA interference has emerged as a major regulatory mechanism in gene silencing in which small non-coding RNAs function as sequence-specific guides. MicroRNAs, a relatively well- characterized class of small non-coding RNAs, are essential for normal development and are implicated in many human diseases. In contrast, the biogenesis and function of piRNAs (Piwi-interacting RNAs), a diverse class of germline-specific small non-coding RNAs, is poorly understood. Recent studies in diverse organisms including fly, zebra fish, and mice have shown that Piwi proteins and piRNAs are required for silencing of transposable elements in the germline. We have found that MOV10L1, a putative RNA helicase, binds to all Piwi proteins and is an essential component of the piRNA pathway. Therefore, MOV10L1 is the only known master regulator of the mammalian piRNA pathway. Disruption of Mov10l1 leads to a loss of piRNAs and meiotic arrest in males. Using our unique Mov10l1 conditional mutant and other genetic models, we plan to: 1) investigate the chromatin integrity and functional competency of MOV10L1/piRNA-deficient round spermatids, 2) elucidate the role of MOV10L1 in the biogenesis and function of pachytene piRNAs, and 3) biochemically characterize the role of the MOV10L1 homologue in piRNA biogenesis in cultured silkworm (Bombyx mori) BmN4 ovarian cell lines. These studies will elucidate the mechanistic basis of piRNA biogenesis and a novel function for MOV10L1 and pachytene piRNAs in maintaining genome integrity during spermiogenesis. These studies will provide insight into etiology, diagnosis, and prevention of human diseases including male infertility and birth defects.