We previously reported genome editing technology-mediated candidate variant knock-in in human cultured cells with a uniform genetic background to confirm that heterozygous ATM mutations, which cause a rare autosomal-recessive disease, ataxia-telangiectasia [A-T; Online Mendelian Inheritance in Man (OMIM): 067585], indeed underlie chromosomal radiosensitivity8. The gene discussed is ATM; the disease is ataxia telangiectasia.