Hedström AK, Hillert J, Olsson T, et al. - Researchers used two Swedish population-based case–control studies, to assess environmental risk factors as well as their interactions with human leucocyte antigen DRB1*15:01 regarding relapsing-onset and progressive-onset multiple sclerosis (MS). They found all environmental as well as lifestyle factors related to risk of MS development apply to both relapsing-onset and progressive-onset disease. Elevated risk of both MS phenotypes was observed in relation to smoking, obesity and Epstein-Barr virus nuclear antigen-1 (EBNA-1) antibody levels, whereas a decreased risk was noted in relation to snuff use, alcohol intake and sun exposure. There were additive interactions between DRB1*15:01 and smoking, obesity, EBNA-1 antibody concentrations and sun exposure, respectively, resulting in elevation of MS risk irrespective of the clinical phenotype. According to findings, both relapsing-onset and progressive-onset MS are affected by the same environmental and lifestyle factors, thus, supporting the notion that the different clinical phenotypes share common underlying disease mechanisms.