ON page 150 of this issue1, Ebers et al. describe the results of a painstaking study of 15,000 patients suffering from multiple sclerosis (MS). Their conclusion is that the aggregation of MS cases within families is determined entirely by genetic factors and is not influenced by shared environmental factors like infection. This vexing genes-versus-environment issue has been convincingly resolved as a result of the authors' comprehensive screening of adoptive first-degree'relatives' of a large, fully ascertained and well studied population of MS patients from fourteen clinics across Canada. In a sense, these findings are not surprising. It is now widely thought that MS is the result of an immunologically mediated, quite possibly autoimmune, process. Susceptibility to MS seems to be determined, in part, by the genes that encode elements of the trimolecular complex involved in recognition by T cells. These include the major histocompatibility complex (MHC), certain T-cell receptor peptide sequence polymorphisms and encephalitogenic peptides of brain protein antigens; also implicated are genes governing elements of the immunologically determined inflammatory process. The high concordance of MS in monozygotic twin pairs and in first-degree relatives of individuals with MS, cited by Ebers et al.', is an expression of these genetic influences.

But although Ebers and co-workers were unable to detect any effect attributable to shared environment, things may not be quite so simple. These authors only addressed the question of MS prevalence among non-biological'relatives' living with an MS case, most of whom do not carry genes for MS susceptibility. But shared environment undoubtedly does play a part in the induction of disease among biological relatives who live with-SOLAR PHYSICS--------------------~