The endometrial surface is required to reject attachment of exogenous undesirable substances such as pathogens, while occasionally shifting properties to allow physical interaction with an embryo to support a successful pregnancy. Kubota et al. reported that MUC1, a potent inhibitor of cellular contact via its large extracellular glycoprotein domain, localizes to the apical surface of bovine endometrium (upper panel, green staining). This MUC1 barrier can be removed in response to steroid signaling at receptive phase and during pregnancy (lower panel), as revealed by immunohistochemistry (p. 386–391). Red signal (pseudo-color) represents DAPI-stained nuclei. Interestingly, MUC1 expression is elevated in the endometrium of cows with a long postpartum interval, which is a known risk for reducing female fertility. Thus, this observation is consistent with the proposed anti-attachment role of MUC1. These results suggest that MUC1 regulation plays a role in endometrial homeostasis and embryo implantation, and its negative correlation with fertility may be used as a diagnostic of reproductive efficiency.