Objectives. The subcellular localization of the breast cancer susceptibility gene product BRCA1 has been controversial. Discrepant results have been reported during the past 3 years, partially because of the unavailability of highly specific reagents for BRCA1 protein. Our objective was to characterize the BRCA1-like immunoreactivity that is detected in human seminal plasma by using monoclonal and polyclonal antibodies that are supposedly specific for BRCA1 protein.

Methods. We used immunologic, chromatographic, and protein sequencing techniques to detect the immunoreactivity of BRCA1 in seminal plasma and to purify and partially identify the immunoreactive species.

Results. We present data indicating that two BRCA1 antibodies, SG-11 and D-20, which were thought to be free of cross-reactivities, strongly interact with proteins present in human seminal plasma. This cross-reactivity is detectable even at seminal plasma dilutions as high as 10⁶-fold, and it is effectively blocked by peptides that capture the binding site of either SG-11 or D-20 antibodies. Purification and characterization of the immunoreactive compound revealed that this consists of a macromolecular complex that contains semenogelins. The D-20 polyclonal antibody was found to cross-react with purified semenogelins I and II; the SG-11 monoclonal antibody appeared to recognize a component of the macromolecular complex that was not semenogelin.

Conclusions. Our data demonstrate that the BRCA1 antibodies SG-11 and D-20 strongly interact with seminal plasma proteins and are not highly specific for BRCA1 protein. It is thus suggested that BRCA1 antibodies should be used with caution until reagents free of interference are developed and evaluated. In light of the very high cross-reactivity of the two antibodies with seminal plasma proteins, we recommend that new BRCA1 antibodies should be examined for cross-reactivity with seminal plasma proteins to verify specificity.