Ectopic production of placental proteins by a variety of nontrophoblastic epithetial tumors is well recognized.
Pregnancy-specific beta-1 glycoprotein (SP-I), a recently described placental protein, has been detected both in the serum and tumors of patients with breast carcinoma. To assess the significance of SP-1 in breast carcinoma, we stained 139 cases of primary breast carcinoma for SP-1 using the immunoperoxidase technique. Overall, 55 (40%) of breast cancers were positive for SP-1; focal positivity was also noted in normal breast epithelium and fibrocystic disease. Both intraductal (32%) and infiltrating duct (43%) carcinomas were more often positive than either in situ (5%) or infiltrating (26%) lobular carcinomas. SP-1 positivity in tumors of infiltrating duct morphology less than 3 cm in diameter, correlated highly (P < 0.01) with the presence of axillary lymph node metastases. The presence of SP-1 in normal breast epithelium and fibrocystic disease and the low rate of positivity in lobular carcinoma casts doubt on the usefulness of SP-1 as a tumor marker. However, these findings suggest that immunopositivity for SP-1 in small infiltrating duct carcinomas may be an indicator of poor prognosis.