Itacitinib (INCB039110; Incyte Corporation, Wilmington, DE) (IUPAC name: 2-[1-[1-[3-fluoro-2-(trifluoromethyl)pyridine-4-carbonyl]piperidin-4-yl]-3-[4-(7H-pyrrolo[2,3-d]pyrimidin-4-yl)pyrazol-1-yl]azetidin-3-yl]acetonitrile) is a novel oral selective JAK1 inhibitor, approved in 2018 by EMA as an orphan drug for the treatment of graft-versus-host disease (GVHD) [214], and currently tested for other clinical indications, including oncologic and immunologic disorders. The gene discussed is JAK1; the disease is graft versus host disease.