In the past year, researchers have identified the genes responsible for X-linked severe combined immunodeficiency (encoding a cytokine receptor protein), X-linked agammaglobulinemia (encoding a cytoplasmic tyrosine kinase) and X-linked hyper IgM syndrome (encoding the ligand for CD40). Although these three genes are completely unrelated, it is of interest that all are lineage-specific genes that are involved in the control of lymphocyte proliferation or differentiation.