1. G2/M checkpoint stringency varied between AML cell lines; U937 cells showed an earlier exit from the checkpoint than KG1a cells after exposure to etoposide. Etoposide caused increased phosphorylation/activation of CHK1 in both cell lines, but the earlier exit by U937 cells was associated with an earlier decrease in CHK1 activation. This evidence concerns the gene CHEK1 and acute myeloid leukemia.