CHEK1 and cancer: In particular, we analyzed phosphorylated histone H2AX (γ-H2AX), which is a well-known marker of double-strand breaks, and replication protein A (RPA) and CHK1, which have previously been implicated in the DDR pathway induced by dl922-947 [27,28] or AZD1775 [44,68,69,70,71,72,73] in other cancer cell types.