ATM and ataxia telangiectasia: Chromosome bands 11q22-q23, the region most frequently lost in NB, contain ATM, the gene mutated in ataxia telangiectasia (AT), an autosomal recessive syndrome characterized by neurodegeneration, oculocutaneous telangiectasia, radiosensitivity, immune deficiency, sterility, strong predisposition to lymphoid cancers and, at the cellular level, cell-cycle checkpoint defects and chromosomal instability.