Janus kinase (JAK) is a family of intracellular tyrosine kinases that transmit information from chemical signals via the JAK/STAT pathway. Molecules such as colony-stimulating factor, growth hormone, and many cytokines are reliant on JAK/STAT signaling for downstream activation of their effects. There are four JAK family members, named JAK1, JAK2, JAK3, and Tyrosine Kinase 2 (TYK2). Some JAK inhibitors act on just one of these members while others will act on two or more. Two JAK inhibitors are currently FDA approved: a JAK inhibitor by the name of ruxolitinib was approved in 2011 for treatment of a rare bone marrow cancer and another, tofacitinib, in 2012 for treatment of rheumatoid arthritis. Several others have reached clinical and preclinical trials for diseases such as psoriasis, inflammatory bowel disease, alopecia, and myelogenous leukemia among others.