SUM149 cells were specifically sensitive to combined GSK126/AZD1390 inhibition leading to a significantly reduced cell viability by 77–79%, compared to vehicle control (p = 0.02) or single agent treatment (GSK126: p = 0.02; AZD1390: p = 0.03), while no effect was observed in CAL120 cells (Fig. 4A), suggesting that the cytotoxic effect by EZH2/ATM inhibition might be specific to BRCA1 mutation status in human breast cancer cells. Here, BRCA1 is linked to breast carcinoma.