Tofacitinib is a second-generation selective Janus kinase (JAK) inhibitor targeting the JAK1 enzyme, which is used to treat several autoimmune diseases, including rheumatoid arthritis (RA), psoriatic arthritis, ankylosing spondylitis, idiopathic arthritis, and ulcerative colitis. The gene discussed is JAK1; the disease is ankylosing spondylitis.