Because IKKε inhibitor was relatively cytostatic compared to CHEK1 inhibitor and showed linear cellular toxicity at the lower concentrations, CI indicated either additive or synergistic effects (CI ≤ 1) at the lower concentration range, but the effect generally became antagonistic (CI > 1) at the higher range in 6 ovarian cancer cell lines. The gene discussed is CHEK1; the disease is ovarian cancer.