BRCA1 and breast cancer: We then determined the IC50 value of GSK126 for all cell clones, showing that 3- to 11-fold lower concentrations of GSK126 were required for a 50% reduction in viability in BRCA1-deficient compared to BRCA1-proficient cells, indicating that BRCA1-deficient mammary tumor cells are more sensitive to EZH2 inhibition (Fig. 1C; KB1P-G3: IC50 = 13.4 μM; KB1P-B11: IC50 = 17.6 μM; KP-3.33: IC50 = 44.9 μM; KP-6.3: IC50 = 148.9 μM).