BRCA1 contains at least two nuclear localization signals and is proposed to be a tumor suppressor protein. 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. BRCA1 protein alters in a qualitative and quantitative manner during cell cycle progression. The amount of BRCA1 protein is highest during S-phase and remains elevated toward G2/M, before it declines in early G1-phase. Inherited loss of BRCA1 function confers an increased susceptibility for both breast and ovarian cancer.