HLA-DRB1 and Epstein-Barr virus infection: Variations in the human leukocyte antigen (HLA) region influence susceptibility to MS with the main effect originating from the Class II DRB1 gene.1 Environmental and lifestyle factors increasing MS risk include smoking,2 low sun exposure,3 low vitamin D status,4 high body mass index (BMI),5,6 and Epstein-Barr virus (EBV) infection.7 High BMI during adolescence and young adulthood, but not at the time of MS onset, has repeatedly been associated with an increased risk of developing MS.5,6 The risk increases in a dose-dependent manner.6