In myeloid cells BTK is phosphorylated on Y551 upon DAP12 activation in a SYK kinase-dependent manner. Y551 is located in the activation loop of BTK, known to be required for activation and kinase activity. Y223 in the SH3 domain of BTK is autophosphorylated, which may also be involved in BTK activation (Ormsby et al. 2010, Rawlings et al. 1996).