MUC1 is a large, highly glycosylated protein expressed on the apical membrane of many epithelial cells. With other members of the mucin family it contributes to the protection and function of mucosal cells. The intracellular part of the protein may also participate in signal transduction pathway, through multiple interactions with intracellular proteins. Overexpression of MUC1 is frequently observed in the majority of epithelial cancers and even in some haematological malignancies. In tumor cells, MUC1 loses apical distribution and is hypoglycosylated. These cancer-associated changes render it antigenic and make it an attractive target for a specific cancer immunotherapy. Several MUC1-based therapeutic cancer vaccines are currently under clinical investigation.