Per Wikipedia, the Janus kinases:
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 beginnings and endings, 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.
Click here to see all 8649669 letters of Day 199 with JAK2 underlined.