BTK: ProductsBruton tyrosine kinase (Btk) is a 75 kDa cytoplasmic protein tyrosine kinase that is widely expressed in hematopoietic cells. Btk is required for B cell receptor signaling and B cell development. Defects in Btk result in X-linked agammaglobulinemia which is characterized by a severely decreased level of circulating antibodies. Like other Tec family kinases, Btk contains a Pleckstrin homology domain, a Tec homology domain, an SH3 domain, an SH2 domain, and a protein kinase domain. Association of the Btk SH2 domain with the B cell linker protein (BLNK) is required for the activation of PLC? by Btk. The autophosphorylation of Tyr223 facilitates the interaction of Btk with activated Syk. Within the SH2 domain (aa 280 - 377), human Btk shares 99% aa sequence identity with mouse and rat Btk.