Of these, 6 patients had common variable immunodeficiency (CVID), 3 had X-linked agammaglobulinemia (XLA) due to loss-of-function mutations in the Bruton tyrosine kinase (BTK), and 1 patient had autosomal recessive agammaglobulinemia (ARA), the etiology of which is unknown. Here, BTK is linked to isolated agammaglobulinemia.