CHEK1 and cancer: The observed synergistic effects of Wee1 and ATR inhibition (71, 115) on cancer cell killing are surely in grant part due to the lowering of the threshold for CDK activation by combining inhibiting the constitutive phosphorylation and preventing checkpoint activation by the ATR/CHK1/Cdc25 axis, as combined AZD1775 and AZD6738 treatment leads to mitotic catastrophe in cancer cells (71).