CHEK1 and cancer: While APE2 had a significant (α = 0.05) positive correlation with PCNA, APE1, XRCC1, PARP1, Chk1, and Chk2 across all 6 cancer types, groupings of DNA repair and DDR genes were found to be correlated with APE2 in different patterns in different cancer types (Figs. 4, and S14–S18).