Muc1 is a heavily O-glycosylated transmembrane protein expressed on most secretory epithelium, including mammary gland and some hematopoietic cells. It is expressed abundantly in lactating mammary glands and overexpressed in >90% breast carcinomas and metastases. In normal mammary gland it is expressed in apical surface of glandular epithelium. In breast cancer Muc1 is overexpressed; is under-glycosylated and the apical localization is lost. Muc1 is transcribed as a larger precursor which is cleaved to form a larger mucin like subunit (265-400kDa) and a smaller subunit (14- 28kDa) noncovalently associated with each other. Transgenic Muc1 has been shown to associate with all four erbB receptors and localize with erbB1 (EGFR) in lactating glands. Muc 1 can act as a ligand for ICAM-1 on HUVEC cells; it can bind beta-catenin , GSK3beta and it associates with Grb2-SOS upon phosphorylation.