BRCA1 IS A CELL-CYCLE-REGULATED NUCLEAR PHOSPHOPROTEIN

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 approximate to 220-k Da BRCA1 protein that is predominantly expressed in the nucleus of bot h normal and neoplastic breast cancer cells. It is a serine phosphopro tein that undergoes hyperphosphorylation during late G(1) and S phases of the cell cycle and is transiently dephosphorylated early after M p hase. We propose that BRCA1 is a phosphoprotein that alters in a quali tative and quantitative manner during cell cycle progression.