Cytarabine induces Chk1 activation in HL-60 cells. HL-60 cells were treated with diluent or the indicated concentrations of cytarabine for 3 hours. After cells were washed, postnuclear lysates were prepared. Equal amounts of protein were electrophoresed and sequentially immunoblotted for antiphospho-Ser345-Chk1 and total Chk1.