Mammary serum antigen (MSA) levels as a marker for breast cancer: a survey of blood donors and patients with cancer.

A murine monoclonal antibody (3E1.2) to human breast carcinoma cells was made. The antibody, which was selected for its ability to react with formalin fixed sections of the immunizing tissue, detects an antigen in the serum of breast cancer patients which we have called Mammary Serum Antigen. Immunoperoxidase staining has shown that there is an increased expression of this antigen on malignant breast epithelium compared to normal breast epithelium and other normal tissues. An assay (a serum inhibition enzyme immunoassay) using this antibody has been established to determine levels of MSA in patients' sera. Two independent studies involving greater than 2,000 blood donors and greater than 400 patients with breast cancer have shown that MSA is elevated in patients with carcinoma of the breast. MSA levels were found to be raised in approx. 2% of blood donors, approx. 55% of patients with localised breast cancer (Stage I and II) and in approx. 85% of patients with advanced breast cancer (Stage III and IV). Raised levels were also found in some patients with benign breast disease and other tumors, however these were generally much lower than those found in breast cancer patients. MSA levels may therefore be of some use for the monitoring of breast cancer patients, and as a diagnostic aid to screen populations for breast cancer.