Since the first description of a patient with recurrent infections and deficiency of immunoglobulins termed “Agammaglobulinemia” by Ogden Bruton in 1952 (1), it took more than 40 years of research, until Bruton tyrosine kinase (Btk) was identified in 1993 as the responsible protein that is deficient in patients with X-linked agammaglobulinemia (2, 3). Here, BTK is linked to isolated agammaglobulinemia.