We have characterized the BRCA1 gene product by using four polyclonal antibodies raised against peptides from four different regions of the protein. The antibodies specifically recognize an approximately 220-kDa BRCA1 protein that is predominantly expressed in the nucleus of both normal and neoplastic breast cancer cells. It is a serine phosphoprotein that undergoes hyperphosphorylation during late G1 and S phases of the cell cycle and is transiently dephosphorylated early after M phase. We propose that BRCA1 is a phosphoprotein that alters in a qualitative and quantitative manner during cell cycle progression.