Tyrphostin AG 490 is a specific and potent JAK-2 protein tyrosine kinase inhibitor. It has been shown to inhibit EGF receptor autophosphorylation, DNA synthesis and cell growth. It induces apoptosis with no deleterious effect on normal hematopoiesis. Tyrphostin AG 490 has been shown to inhibit the growth of leukemic cells in vitro and in vivo. This compound is a valuable tool for studying the cellular role of JAK kinases in signal transduction.