 Mutations in the BTK gene are implicated in the primary immunodeficiency disease X-linked agammaglobulinemia (Bruton's agammaglobulinemia); sometimes abbreviated to XLA and selective IgM deficiency.Patients with XLA have normal pre-B cell populations in their bone marrow but these cells fail to mature and enter the circulation.The Btk gene is located on the X chromosome (Xq21.3-q22).