Project Summary Germ cells undergo extensive epigenetic reprogramming during development. During this critical period, retrotransposons are reactivated due to genome-wide erasure of DNA methylation and are re-silenced by de novo DNA methylation. Retrotransposons, mainly LINEs, SINEs, and endogenous retroviruses (collectively referred to as junk DNA), occupy 40% of the mammalian genome. Although retrotransposons play an important role in genome evolution, their mobilization could be detrimental to genome integrity. Multiple epigenetic mechanisms are responsible for silencing retrotransposons in the germline: DNA methylation, repressive histone modification, small RNAs, and heterochromatinization. Given the extreme abundance of the retrotransposons and the paramount importance of germline genome integrity, novel mechanisms for retrotransposon silencing may exist. In support, we have found that TEX15, a germ cell-specific 3059-aa protein with a newly identified functional domain, is required for meiosis and male fertility, and is a novel epigenetic regulator essential for retrotransposon silencing. Based on these data, we hypothesize that TEX15 is a novel germ cell-specific regulator of retrotransposon activation and that meiotic collapse is the ultimate “fail-safe” mechanism for preventing transmission of male germ cells in which retrotransposons are inordinately activated. In this project, we will 1) determine the TEX15-mediated epigenetic landscape during male germ cell development in mouse using genomic approaches, 2) elucidate molecular mechanisms underlying TEX15 function, and 3) screen for retrotransposon activation and utilize whole exome sequencing to identify genetic mutations that compromise epigenetic silencing of retrotransposons in testis biopsies from azoospermic men. Together, these studies will identify novel factors in the silencing of retrotransposons and provide essential insights into the etiology of male infertility in humans.