GM-CSF is elevated in the serum (5) and synovial fluid (6, 7) of RA patients, is expressed in rheumatoid synovium (8), its in vivo administration can induce RA exacerbation (9) while its inhibition has beneficial effects in animal models of arthritis (10) and patients with RA (11–14). The gene discussed is CSF2; the disease is Arthritis.