ABSTRACT
I. INTRODUCTION Although there is agreement that exogenous factors are very relevant in the pathogenesis of multiple sclerosis (MS), the genetic aspects have been studied more since the second edition of this book was published in 1996. The compelling data from twin studies leave no doubt that there is a strong familial factor in MS, with a concordance rate for monozygotic twins of nearly 40% (although this necessarily means that there is more than a 60% discordance)—a figure increasing with duration of follow-up. The rates in fraternal twins and nontwin siblings are similar at about 3%, translating into a prevalence of 2000 per 100,000. This high rate, if only partly the product of genetic influences, must spring largely from environmental effects if it is supposed that fraternal twins are frequently in each other’s company. Studies of large enough numbers of twins separated from an early age will soon be available, and the ongoing Canadian Collaborative Study on Genetic Susceptibility to MS has shown that among the adopted siblings of MS patients, the prevalence of MS is much the same as in the general population (1). Hence, one persists in examining the epidemiology of the disease in order to draw inferences about its possible causes. In the 4 years since the second edition of this book, some new data have been produced; these are examined here, the overall format being maintained.
