BRCA1 expression is induced before DNA synthesis in both normal and tumor-derived breast cells

Insight into the function of the BRCAI tumor suppressor gene may be gained by studying its regulation. In this study, the expression of BRCAI was examined as a function of the cell cycle in normal and tumor-derived breast epithelial cells. Cells arrested in G0 or early in G1 contained low levels of BRCAI mRNA. After release, populations of cells reached maximal levels of BRCAI in late G1 and S phase. Induction of BRCAI was shown to occur before the onset of DNA synthesis by synchronizingcells at the G1-S boundary. Levels of the BRCAI protein were regulated in a similar manner. No difference was observed between primary cultures of normal mammary epithelial cells and immortalized tumor-derived cell lines. These results suggest that BRCAI may function at the G1-S checkpoint.