BTK and Bruton-type agammaglobulinemia: Bruton’s tyrosine kinase (Btk), also known as agammaglobulinemia tyrosine kinase (TK), is a member of the Tec kinase family, initially identified as a defective protein in human X-linked agammaglobulinemia (XLA) in 1993 by Vetrie and coworkers [1,2].