The molecular basis for three well-defined X-linked diseases has recently been identified. In X-linked agammaglobulinemia, the gene encoding a novel cytoplasmic tyrosine kinase (btk) expressed by B cells is defective. This B-cell-specific kinase belongs to a new subfamily of tyrosine kinases. The molecular defect in X-linked hyper IgM affects the gene encoding the CD40 ligand (CD40L, gp39) on T cells. This protein binds to its natural receptor, CD40, expressed constitutively by B cells. The ligand-receptor interaction initiates B-cell proliferation and isotype switching. The molecular defect in X-linked severe combined immunodeficiency disease has been assigned to the gene encoding the y chain of the interleukin-2 receptor (IL-2Rγ), which is constitutively expressed by T cells and is involved in the formation of high and intermediate affinity IL-2R complexes. IL-2R-γ is responsible for the failure of X-linked severe combined immunodeficiency disease T and B lymphocytes to respond to IL-2–dependent signals.