Susceptibility to rheumatoid arthritis and systemic lupus erythematosus has been linked to a region on chromosome 2q. Fine-mapping of this locus, in patients with rheumatoid arthritis and in patients with lupus, traces the association to a single variant of the gene STAT4, which encodes a transcription factor activated by cytokines. Fine mapping of a region on chromosome 2q, in patients with rheumatoid arthiritis and in patients with lupus, traces an association to a single variant of the gene STAT4, which encodes a transcription factor activated by cytokines. Rheumatoid arthritis is the most common cause of adult inflammatory arthritis and is associated with considerable disability and early mortality. 1 Studies of twins clearly show a genetic contribution to disease susceptibility, 2 and the siblings of patients with seropositive, erosive rheumatoid arthritis have an estimated risk of developing the disease of between 5 and 10 times that of the general population. 3 The highly polymorphic HLA region is a major contributor to genetic risk of rheumatoid arthritis. 4 Several other genes associated with more modest risks have recently been identified, including the Arg620→Trp variant of the intracellular phosphatase gene PTPN22 . 5 , 6 However, . . .