JANEX-1 (WHI-P131), a selective Janus kinase 3 (JAK3) inhibitor, has been shown to delay the onset of diabetes in the NOD mouse model. It is a cell-permeable, reversible, potent, ATP-competitive, and specific inhibitor of JAK3 (IC50 = 78 uM); has no effect on JAK1, JAK2, or Zap/Syk or SRC tyrosine kinases. JANEX-1 has a potent inhibitory effect on cytokine-induced β-cell damage. JANEX-1 has a therapeutic potential in the treatment and prevention of type 1 diabetes. JANEX-1 induced apoptosis in JAK3-expressing human leukemia cell lines NALM-6 and LC1. JANEX-1 inhibited the clonogenic growth of JAK3-positive leukemia cell lines DAUDI, RAMOS, LC1;19, NALM-6, MOLT-3, and HL-60 (but not JAK3-negative BT-20 breast cancer, M24-MET melanoma, or SQ20B squamous carcinoma cell lines) in a concentration-dependent fashion. Potent and specific inhibitors of JAK3 such as JANEX-1 may provide the basis for the design of new treatment strategies against acute lymphoblastic leukemia, the most common form of childhood cancer. JANEX-1/WHI-P131 also showed potent anti-inflammatory activity in several cellular and in vivo animal models of inflammation, including mouse models of peritonitis, colitis, cellulitis, sunburn, and airway inflammation with favorable toxicity and pharmacokinetic profile. JANEX-1 may prove useful to prevent or alleviate the symptoms of endometriosis (EMS).