Investigators at the University of California in San Francisco are recruiting people with MS for an international study of the gut microbiome – the population of bacteria in the gut – in MS. They are seeking people with primary progressive MS nationwide (there is no need for onsite visits), as well as people with any other type of MS who can make a one-time visit to San Francisco, New York, Boston or Pittsburgh. The overall purpose of these studies is to investigate the potential role of gut bacteria in MS.