MUC-1 is a large cell surface mucin glycoprotein expressed by most glandular and ductal epithelial cells and some hematopoietic cell lineages. MUC-1 is secreted from tumor cells. MUC-1 antibody stains cell membranes, but also the cytoplasm of most epithelial cell types. It is expressed abundantly in lactating mammary glands and over-expressed in >90% breast carcinomas and late-stage epithelial ovarian cancers. Aberrant cytoplasmic and membranous localization of MUC-1 expression has been associated with poor patient outcome. Adenocarcinomas are generally positive while squamous carcinomas and non-epithelial malignancies are negative.