HLA-C and multiple sclerosis: Variation within the major histocompatibility complex (MHC) has the greatest impact on individual risk [odds ratio (OR) associated with DRB1*1501 3.1 and 6.2 for heterozygous and homozygous carriage, respectively].2,3 Genome-wide association studies (GWAS) of multiple sclerosis susceptibility have demonstrated at least 200 risk alleles outside the MHC locus, each with a small incremental effect (OR per allele ≤1.3).2 There is no convincing evidence for monogenic forms of multiple sclerosis in the general population.4