This session will focus on genetic factors determining the susceptibility to MS. While this symposium will be open to all, topics will be discussed at an intermediate to advanced level. MS is one of the success stories of modern genetics. Prof. Patsopoulos will introduce the concept of genetic heritability and discuss the genome-wide association efforts in MS over the past decade. While identifying statistical associations usually constitute the first step, they do not automatically translate into a biological explanation of disease pathogenesis. Prof. Baranzini will build from the previous information and elaborate on how genetic susceptibility on various regions of the genome can coalesce to confer risk in specific cellular pathways. In addition, the role of gut bacteria will be discussed as a risk factor in MS. Dr. Harroud will discuss the concept of Mendelian randomization as a strategy to eliminate confounding factors and focus on causal genetic associations. Finally, most of our knowledge of MS genetics is derived from the study of European populations. However, it is not yet known how much of the risk is shared by other ethnicities. Dr. Amezcua will show evidence of shared and unique genetic risk profiles in Hispanic and African American populations and compare that with the genetic risk identified for Europeans.