JAK1 and myelofibrosis: Momelotinib, a JAK1/JAK2/activin A receptor type 1 (ACVR1) inhibitor approved specifically for the treatment of patients with myelofibrosis who have anemia [7], has demonstrated clinically meaningful and durable improvements in anemia, splenomegaly, and symptoms in both JAK inhibitor-naive and -experienced patients with myelofibrosis across 3 phase 3 trials [8–10].