The human milk fat globule (HMFG) membrane contains several glycoproteins that have been referred to as breast differentiation antigens and that are expressed in normal breast, breast tumors, breast tumorderived cell lines, and are found in breast cancer patient serum. These antigens include a high molecular weight mucin and several smaller components including Mr 150,000; 70,000; and 46,000 glycoproteins. We have used 2 monoclonal antibodies (McR2 and Mc13) that bind the Mr 70,000 component of HMFG to immunoscreen a λgt11 expression library prepared from human lactating breast tissue. We report here the sequence of a complementary DNA clone (BA70-1) that codes for a peptide that binds both McR2 and Mc13 but not monoclonal antibodies to the breast mucin or other components of HMFG. A 1.8-kilobase RNA was detected in 9 of 9 breast tumor cell lines using 32P-labeled BA70-1 as probe. The BA70-1 RNA was highly expressed in 6 of 9 cells lines of breast and several other carcinomas lines compared with a lymphoblastoid cell line (Raji). The BA70-1 complementary DNA sequence has no extensive homology with previously reported sequences including the high-molecular weight mucin complementary DNA. Since the Mr 70,000 molecule appears to be associated with the breast mucin by disulfide bonds, its study could help elucidate the structure of this latter complex and how it is organized in the cell membrane, and prove useful in diagnosis and therapy of breast cancer.

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This work is supported by NIH Grants CA39932, CA42767, and RR05929.

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