The TEC family is ancient and constitutes the second largest family of cyto-plasmic tyrosine kinases. In 1993, loss-of-function mutations in theBTK gene were reported as the cause of X-linked agammaglobulinemia. Of all the existing 90 tyrosine kinases in humans, Bruton’s tyrosine kinase (BTK) is the kinase for which most mutations have been identified.