BTK and isolated agammaglobulinemia: X-Linked Agammaglobulinemia (XLA, OMIM entry 300,755) is a classic model of predominantly antibody deficiencies caused by mutations in Bruton’s Tyrosine Kinase (BTK) gene (OMIM #300300) with a reported incidence rate of 1/100,000 or 1/200,000 live births.