In normal breast tissue, 304-1A5 labels breast ductal and lobular epithelial cells. In tumor cells, it is reactive with all types of breast adenocarcinoma regardless tumor differentiation and types. Adenocarcinomas from other organs rarely express mammaglobin. Mammaglobin is a breast-associated glycoprotein distantly related to secretoglobin family that includes human uteroglobin and lipophilin. Unlike other secretoglobin family members, mammaglobin mRNA expression is breast specific, which has been shown to be a very sensitive marker of occult breast cancer cells in sentinel lymph nodes and peripheral blood. By paraffin immunohistochemistry, the overall sensitivity of mammaglobin for breast cancers was reported about 80%. When combined with other breast-restricted markers such as GCDFP-15, an overall sensitivity of 84% could be achieved. Mammaglobin can play a contributing role in the identification of primary sites of carcinomas presenting at metastatic sites.