The indispensable role of prolactin in mediating lactation-induced infertility in the mouse is surprising, given the consensus of much work in other species concluding that other factors may be more important (see McNeilly, 2001; McNeilly, 1997; Tay et al., 1996; Díaz et al., 1989; Woodside and Jans, 1995). The gene discussed is PRL; the disease is Infertility.