Experimental studies of human implantation in which embryos hatch and attach to endometrial epithelial cells in monolayer culture have shown that Mucin (MUC1) disappears from a small area of cells that surrounds the attached embryo. The term 'marker' is used advisedly because, while the inclusion criteria include the ability to function as an adhesion molecule, as well as presence at the implantation site, in vivo functional (IVF) evidence is so far mostly lacking. Heparin-binding EGF-like growth factor (HB-EGF) has been suggested to play two different roles: in addition to accelerating embryo development, it has a membrane-bound variant that can mediate intercellular attachment by binding to ErbB4 and heparan sulphate proteoglycan, both of which are present on the blastocyst. Critically, the biochemical dialogue between the embryo and the endometrium that determines endometrial receptivity at the attachment site cannot emerge from an analysis of either one alone.