Protein tyrosine kinase (Ptk) inhibitors are potential antiproliferative agents for diseases caused by the hyperactivity of Ptks. Tyrphostins are a class of antiproliferative compounds which act as Ptk blockers. Leukemic cells from patients in relapse have constitutively activated JAK2 Ptk. AG-490 is an inhibitor of JAK2 activity and selectively blocks leukemic cell growth in vitro and in vivo by inducing programmed cell death, with no deleterious effect on normal hematopoiesis. AG-490 almost completely blocks growth of all pre-B acute leukemia (ALL) cells at a concentration of 5 µM.