JAK3 and juvenile myelomonocytic leukemia: The Human JAK family can be classified into 4 members including JAK1, JAK2, JAK3 and Tyk2.134,135 JAK2 is known to mediate important physiological functions such as cell proliferation.136,137 Overexpression of JAK3 is also associated with juvenile myelomonocytic leukemia.138 Tofacitinib was the first approved JAK inhibitor, after which various JAK inhibitors were developed.