PIWIL1 and male infertility: It was found that yak male infertility is associated with promoter hypermethylation and the transcriptional silencing of the piwi/piRNA pathway-associated genes TDRD1, PLD6, PIWIL1, DDX4, MAEL, FKBP6, and TDRD5, and the transcriptional silencing of these genes drives the reduced production of piRNAs in the pachytene stage of cattle germ cells, ultimately resulting in the failure of germ cell development (Figure 4) [114].