Genes encoding functional components of the immune system are enriched within loci associated with the development and natural history of rheumatoid arthritis; protein products pathogenetically implicated include cytokines themselves [IL-2, IL-21, G-CSF and granulocyte macrophage-colony stimulating factor (GM-CSF)], their receptor components [for IL-6, IL-20, interferon (IFN)-γ and IL-2] and elements of their downstream signalling machinery (e.g. TYK2, STAT4 and TNFAIP3). Here, IL20 is linked to rheumatoid arthritis.