The study investigates the role of Piwi family proteins, specifically MILI and MIWI2, in the regulation of DNA methylation and piRNA expression in fetal male germ cells. The authors found that both MILI and MIWI2 are essential for maintaining DNA methylation in the regulatory regions of retrotransposons Line-1 and IAP. In the absence of these proteins, DNA methylation is impaired, leading to increased retrotransposon expression. Additionally, the study reveals that the composition of piRNAs in fetal male germ cells differs from that in neonatal and adult germ cells, suggesting a distinct mechanism of piRNA biogenesis during fetal development. The results indicate that MILI and MIWI2 play crucial roles in establishing de novo DNA methylation and regulating piRNA expression, which are critical for maintaining genomic stability and preventing retrotransposon activation.The study investigates the role of Piwi family proteins, specifically MILI and MIWI2, in the regulation of DNA methylation and piRNA expression in fetal male germ cells. The authors found that both MILI and MIWI2 are essential for maintaining DNA methylation in the regulatory regions of retrotransposons Line-1 and IAP. In the absence of these proteins, DNA methylation is impaired, leading to increased retrotransposon expression. Additionally, the study reveals that the composition of piRNAs in fetal male germ cells differs from that in neonatal and adult germ cells, suggesting a distinct mechanism of piRNA biogenesis during fetal development. The results indicate that MILI and MIWI2 play crucial roles in establishing de novo DNA methylation and regulating piRNA expression, which are critical for maintaining genomic stability and preventing retrotransposon activation.