The results of most studies (Table 1) on basal and stimulated prolactin serum levels suggest that some infertile women with endometriosis exhibit a greater capacity for basal and stimulated prolactin secretion than normal fertile women, where the consequence is more severe luteal phase defects as a result of the association of hyperprolactinaemia and endometriosis. The gene discussed is PRL; the disease is hyperpituitarism.