X-linked agammaglobulinemia (XLA; OMIM 300300), first described in 1952 (1), represents the prototype of primary B cell deficiencies caused by mutations of the Bruton's tyrosine kinase (BTK) gene, a member of the Tec family of kinases localized on Xq21.3–Xq22, in the majority of male patients presenting with agammaglobulinemia (2). The gene discussed is BTK; the disease is B cell deficiency.