The BTK gene is located on the X chromosome (Xq21.33-Xq22) and was first described in the setting of X-linked agammaglobulinemia, a disorder characterized by a lack of maturation and development of B lymphocytes and gammaglobulins due to a mutation in BTK [1,2]. The gene discussed is BTK; the disease is Bruton-type agammaglobulinemia.