Pinpointing the expression of piRNAs and function of the PIWI protein subfamily during spermatogenesis in the mouse

► piRNAs are specifically present in germ cells in the testis, especially in spermatocytes. ► piRNAs and PIWI proteins are present in both the cytoplasm and nucleus. ► In the cytoplasm, they are enriched in the chromatoid body. ► In the nucleus, they are enriched in the dense body. ► Spermatocytes without PIWI proteins are arrested at the pachytene stage.