IL-23 is produced by dendritic cells and macrophages and is required for the survival and expansion of pro-inflammatory Th17 cells, which by virtue of their production of IL-17 are associated with the pathogenesis of autoimmune inflammatory disorders, such as multiple sclerosis, rheumatoid arthritis, psoriasis and inflammatory bowel disease15, 16, 17, 18. Here, IL23A is linked to multiple sclerosis.