PIWIL1 and testicular germ cell tumor: They possess 2ʹ-O-methyl-modified 3ʹ ends and form complexes with PIWI proteins, in contrast to miRNAs and siRNAs which pair with AGO proteins.392–394 The PIWI/piRNA complex is known for regulating transposon silencing and reproductive development by controlling gene expression at the transcriptional or post-transcriptional level.395,396 PIWI protein/piRNA deletion or disruption can reactivate transposons, potentially leading to germ cell tumors.