A single female-specific PIWI-interacting RNA (piRNA) determines sex in the WZ sex determination system of the silkworm Bombyx mori, in which males have two Z sex chromosomes and females have one Z and one W sex chromosome. This W-chromosome-encoded piRNA is produced from a precursor termed Fem. In female embryos, the inhibition of signalling by Fem-derived piRNA induced the production of male-specific splice variants of Bmdsx, the RNA products of which act further downstream in silkworm sex development. The researchers identified a target gene of Fem-derived piRNA on the Z chromosome, which they called Masc. Silencing of Masc is needed for the production of female-specific isoforms of Bmdsx in female embryos, whereas in male embryos the Masc protein regulates dosage compensation and masculinization.