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The Janus kinases or JAKs are proteins which act as cytoplasmic tyrosine kinases and link cytokine signaling from membrane receptors to signal transducers and activators of transcription (STAT) transcription factors.
As these molecules are eponyms, the Janus component of the name is capitalized, i.e. "Janus kinase" rather than "janus kinase". The short names for the genes and their protein products are the same 2. Confusingly the first JAK to be discovered is actually called TYK2, and perhaps logically it should be called JAK1, but one can imagine all the problems that would ensue by renaming it!
There are four JAK proteins in humans 1,2:
expression of JAK1 mutations in cancer cells is thought to enable individual cells to contract and potentially allowing them to escape their tumor and metastasize
expressed in hematopoietic and epithelial cells
mutation of the TYK2 gene has been associated with hyperimmunoglobulin E syndrome (HIES)
History and etymology
Janus was the Roman god of starts and ends, and hence of transitions, with no Greek equivalent. Traditionally, he was depicted with two faces, looking in diametrically opposing directions.
In 1991, Andrew Wilks, an Australian biochemist thus applied the Janus name to these novel molecules because of their two near-identical catalytic domains 2-4.