,2 Polymorphisms in TYK2 are associated with a reduced risk of several autoimmune diseases, including T1D, ulcerative colitis, and rheumatoid arthritis.3, 4, 5 Notably, a polymorphism causing a missense mutation in TYK2, leading to decreased function, is associated with protection against T1D.4 Here, TYK2 is linked to ulcerative colitis.