This together with the secretion of glycodelins (placental protein, PP14) by endometriotic cells (Okamoto et al., 1991) and the subsequent decrease in Natural Killer cell activity (Oosterlynck et al., 1991; Kanzaki et al., 1992) plays a role in the immuno-tolerance towards endometriosis. The gene discussed is PAEP; the disease is endometriosis.