Despite a long-term belief in the field that the affected molecule (ATM, a nuclear kinase activated by double-strand breaks or oxidative stress, which phosphorylates several targets including SMARCAL1) was involved in TCR or BCR signaling and thus caused lymphopenia and immunodeficiency, we showed that immortalized T cells derived from patients carried the causing mutations and thus preserved the sensitivity to ionizing radiation of ataxia telangiectasia cells, but did not show any intrinsic immune functional defects when stimulated through their TCR. The gene discussed is ATM; the disease is immune system disorder.