The recent discovery that SOX9 and SSEA1 are preferentially expressed in basalis epithelial cells (Valentijn et al., 2013) provided us with a means to test Leyendecker’s theory and we have confirmed that women with endometriosis have an increased number of basalis-like cells in the secretory functionalis layer that will subsequently be shed at menstruation. The gene discussed is FUT4; the disease is endometriosis.