A selective inhibitor of JAK3 with an IC50 value of 78 µM that does not affect the enzymatic activity of JAK1, JAK2, or other protein tyrosine kinases (IC50 ≥350 µM); induces apoptosis in JAK3-expressing human leukemia cell lines NALM-6 and LC1;19, but not in melanoma or squamous carcinoma cell lines.