Janus kinase (JAK) is a family of intracellular, nonreceptor tyrosine kinases that transduce cytokine-mediated signals via the JAK-STAT pathway. They were initially named "just another kinase" 1 and 2 (since they were just two of a large number of discoveries in a PCR-based screen of kinases,) but were ultimately published as "Janus kinase". The name is taken from the two-faced Roman god of doorways, Janus, because the JAKs possess two near-identical phosphate-transferring domains. One domain exhibits the kinase activity, while the other negatively regulates the kinase activity of the first.
Read more about Janus Kinase: The Janus Kinase Family, General Functions of The JAK Family, The Structure of JAKs