In HL60 (human promyelocytic leukemia cell line), KM12-Luc (human colon cancer cell line), SK-BR-3-Luc (human breast cancer cell line) and HCT15 (human colorectal adenocarcinoma cell line), pretreatment with XL413 reduced HU-induced Chk1 S317 phosphorylation by 32, 25, 24% and 61%, respectively. This evidence concerns the gene CHEK1 and acute promyelocytic leukemia.