Among the mPRs, mPRα is the most studied and mediates numerous P4 effects in fish (Zhu et al., 2003b; Tubbs and Thomas, 2009; Dressing et al., 2010), but also has effects on human breast cancer cells (Dressing et al., 2012) and GnRH release from rodent GnRH neurons (Sleiter et al., 2009). Here, GNRH1 is linked to breast carcinoma.