Momelotinib, a JAK1, JAK2, and activin A receptor type 1 (ACVR1) inhibitor, became the first and, to date, only treatment indicated for patients with myelofibrosis and anemia when initially approved by the US Food and Drug Administration in September 2023, with subsequent approvals by the European Commission, the UK Medicines and Healthcare products Regulatory Agency, and the Ministry of Health, Labour and Welfare of Japan [20,21,22,23]. The gene discussed is JAK1; the disease is anemia (phenotype).