Btk (Bruton tyrosine kinase also known as BPK or ATK) is a member of the TEC family of non-receptor protein tyrosine kinase which includes Bmx, Itk and TEC. Btk is composed of a pleckstrin domain, an SH3 and SH2 domain, a catalytic kinase domain and a TEC homology domain (1). BTK plays a crucial role in B cell activation and development (2). Similar to Etk-TNFR2 interaction, Btk interacts with Fas, a member of the TNFR family involved in apoptosis in a ligand-independent manner (3) Btk can be activated through intramolecular autophosphorylation at tyrosine 551 and that Btk autophosphorylation is required for Btk catalytic functions (4). Mutations in the Btk gene have been linked to severe developmental blocks in human B-cell ontogeny leading to X-linked agammaglobulinemia (XLA). Patients with this disease are unusually prone to bacterial infection but not to viral infection (5).

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