The finding of numerous SSEA1+ cells in the functionalis of eutopic endometrium of women with endometriosis also suggests their delayed differentiation into SSEA1- functionalis epithelial cells given the recent descriptions of a potential hierarchy of epithelial cells from the basalis through the functionalis (Valentijn et al., 2013; Nguyen et al., 2017; Tempest et al., 2018; Tempest et al., 2018). Here, FUT4 is linked to endometriosis.