MUC-1 antigen is a mammary-type apomucin, a high molecular weight transmembrane glycoprotein, of which the extracellular domain is formed by a repeating 20 amino acid sequence (in tandem) with a high content of serine and threonine on which are O-linked carbohydrate chains. MUC-1 synthesis and secretion are features of glandular epithelial tissues; MUC-1 is overexpressed in lactating breast and in breast, ovary, lung, and prostate malignancies.