BTK/ITK are activated in a two step model. In the first step they are recruited to the membrane by binding to PIP3 or, alternatively with other binding partners like SLP-76. Once at the membrane SYK or Src-kinases in the vicinity phosphorylates Y551 (Y512 in ITK) in the activation loop of the catalytic domain of BTK to fully activate it (Rawlings et al. 1996, Park et al. 1996, Kawakami et al. 1994).