[en] Multiple sclerosis (MS) is a chronic neurological disease of the central nervous system (CNS) characterized by demyelination associated with progressive disability. The mechanisms underlying the pathogenesis of MS remain a mystery. The highly pleiotropic syndrome known as ataxia telangiectasia (A-T) overlaps with MS in that it also presents with demyelination in the CNS. Whether demyelination in MS or in A-T is initiated through neuronal degeneration or immune dysfunction is not yet known. However, unlike MS, the underlying cause of A-T is known to result from mutations in the A-T gene (ATM) that often result in the complete loss of ATM protein and loss/gain of function. ATM is implicated in neurological degeneration, particularly in the cerebellum, cellular apoptosis, immunodeficiency, double stranded deoxyribonucleic acid (DNA) rejoining, VDJ antibody recombination, tumour suppression, particularly T-lymphoid malignancies, signal transduction, cell-cycle control and cellular radiohypersensitivity. In this study, we describe a case of MS in a family with cellular radiosensitivity and abnormally low postinduction levels of the ATM protein. Defective DNA repair/rejoining may impact on autoimmunity. Copyright (2002) Blackwell Science Pty Ltd