“This video reveals the research on identical twins with multiple sclerosis. When the effect of genes is removed, the importance of other environmental factors is apparent. What is the risk of MS if your identical twin has it? When identical twins both have MS, is the disease course and severity the same? When twins are discordant (one has MS and the other doesn’t), what are their observable differences? I also tell the story about 4 of my patients, two sets of identical twins.”