Elimination of the male reproductive tract in the female embryo is promoted by COUP-TFII in mice

The makings of the reproductive tract Every embryo, regardless of its sex, contains both male and female primitive reproductive tracts before sexual differentiation. To establish a sex-specific reproductive system, female embryos need to remove the components of male tracts. The general consensus contends that removal of the male tracts occurs by default, a passive outcome owing to a lack of testis-derived androgens. Working in mice, Zhao et al. discovered that this process instead was actively promoted by the transcription factor COUP-TFII (see the Perspective by Swain). Without the action of this factor, embryos retained male reproductive tracts, independently of androgen action. These findings unveil unexpected mechanisms underlying the sexually dimorphic establishment of reproductive tracts. Science , this issue p. 717 ; see also p. 648