PIWIL2 and male infertility: In mice, the deficiency in genes involved in piRNA biogenesis, such as Piwil2 (also called Mili), Piwi4 (Miwi2), Ddx4 (Mvh), and Pld6 (Miotopld) leads to retrotransposon de-repression in germ cells and male infertility (Chuma and Nakano, 2013), indicating the important role of piRNAs in male germ cell development and the maintenance of genomic integrity.