BTK and agammaglobulinemia: BTK is a non receptor, cytoplasmic tyrosine kinase in the Src subfamily encoded by the BTK gene on the X chromosome.1,2 When mutated, the BTK gene confers B-cell development deficits with x-linked immunodeficiency in mice and agammaglobulinemia, the virtual absence of all B cells and immunoglobulins in humans.