Mucins, best known as the major constituent of mucus, are a family of highly glycosylated, high molecular weight (> or = 200 kDa) glycoproteins present on the surface of human epithelial cells. MUC1 has the features of an integral membrane protein. It has an extracellular tandem repeat domain that forms the major part of the core protein, and results in a highly repetitive structure, which is extremely immunogenic. In the protein there is also a proteolytic cleavage site reported in the proximal extracellular domain. The secreted form of MUC1 lacks the cytoplasmic tail, but it is not clear whether this results from alternative splicing or proteolysis and release of the free extracellular domain. The locus of the MUC1 gene is on band 21 of the long arm of chromosome 1 (1q21). Anti-adhesion properties of this mucin are probably the result of the unique structure of the molecule. In mouse uterine epithelium, the homologue MUC1 is regulated with reduced expression in the implantation period, but in humans, expression is high during the peri-implantation period. MUC1 may inhibit the interaction between trophoblast and apical epithelium adhesion molecules at the time of implantation, giving the possibility of forming a uterine barrier for implantation.