These findings strongly demonstrate that JAK3 can serve as a logical target for therapeutic intervention in the hematopoietic malignancies with activating alleles of JAK3. In contrast to the role of gain-of-function of JAK3 in the pathogenesis of hematopoietic malignancies, JAK3 deficiency in mice and human causes immunodeficiency, indicating the pivotal role of JAK3 in the immune system [6,38,39]. The gene discussed is JAK3; the disease is immune system disorder.