We undertook the present study to investigate possible role of MRP4 in regulation of Wnt/β-catenin signaling in the endometrium and discovered unexpectedly that MRP4 acts in a manner independent of its PGE2-transporter function, interacting with and stabilizing β-catenin, to sustain the Wnt/β-catenin signaling for endometrial receptivity during embryo implantation, as well as in pathogenic transformation of the endometrium, i.e. endometriosis and endometrial cancer. The gene discussed is ABCC4; the disease is endometriosis.