Quercetin induced the activation of ERK, and the inhibition of ERK by an ERK inhibitor abolished the quercetin-induced cell apoptosis in HL-60 cells, as confirmed by the activation of caspase-8, caspase-9, caspase-3, PARP cleavage, and mitochondrial membrane depolarization in these AML cells. Here, CASP9 is linked to acute myeloid leukemia.