Purity >98%. Janus kinase 3 (JAK3) is a cytoplasmic protein tyrosine kinase that functions in the initiation of cytokine-triggered signal transduction by catalyzing the phosphorylation of signal transduction and activators of transcription (STATs). JAK3 is abundantly expressed in normal lymphoid cells as well as human lymphoblastic and myeloblastic leukemia cells. WHI-P131 selectively inhibits the tyrosine kinase activity of JAK3 with an IC50 value of 78 μM without affecting the enzymatic activity of JAK1, JAK2, or other protein tyrosine kinases (IC50 =350 μM). WHI-P131 induces apoptosis in JAK3-expressing human leukemia cell lines NALM-6 and LC1;19, but not in melanoma or squamous carcinoma cell lines.