Conclusive molecular diagnoses resulted in clinically important changes including targeted therapies and curative HSCT for patients with X-linked lymphoproliferative syndrome (XIAP), X-linked hyper IgM syndrome (CD40L), severe combined immune deficiency (ADA) and subcutaneous panniculitis-like T-cell lymphoma (HAVCR2). The gene discussed is CD40LG; the disease is T-cell non-Hodgkin lymphoma.