The CD8+ T cell lymphopenia and decreased T cell proliferative responses to mitogens suggested the presence of zeta chain-associated protein of 70 kiloDaltons (ZAP70) deficiency, although the presence of hypomorphic mutations in severe combined immunodeficiency disease (SCID)-causing genes or a novel genetic defect could not be immediately excluded as possible explanations (5–8). This evidence concerns the gene ZAP70 and severe combined immunodeficiency.