Etanercept (trade name Enbrel®) is a protein drug used to treat autoimmune diseases by adsorbing tumor necrosis factor (TNF; a soluble inflammatory cytokine). Etanercept is a fusion protein produced through expression of recombinant DNA by linking the extracellular ligand-binding portion of TNFRSF1B to the Fc component of human immunoglobulin G1 (IgG1). It reduces the effect of naturally present TNF, functioning as a decoy receptor that binds to TNF. Etanercept is indicated for the treatment of moderate to severe rheumatoid arthritis (RA), psoriatic arthritis, ankylosing spondylitis, and moderately to severely active polyarticular juvenile idiopathic arthritis. Serum concentration of Enbrel® may predict some clinical outcome during maintenance therapy.