ACVR1 (activin A receptor type I) is a bone morphogenetic protein (BMP) type I receptor that functions as a serine/threonine kinase involved in diverse developmental processes including bone, heart, cartilage, nervous, and reproductive system development 1. Mechanistically, ACVR1 forms heterotetrameric complexes with type II receptors (AMHR2, ACVR2A, ACVR2B), and upon ligand binding (BMP7, GDF2/BMP9), type II receptors transphosphorylate ACVR1's intracellular domain, activating its kinase activity to phosphorylate SMAD1/5/8 proteins that transduce canonical BMP signaling 2. ACVR1 also suppresses TGF-β/activin pathway signaling and can activate non-canonical pathways such as p38 MAPK. Additionally, ACVR1 regulates iron homeostasis through hepcidin expression 3. Disease relevance is substantial: germline activating mutations in ACVR1 cause fibrodysplasia ossificans progressiva (FOP), characterized by progressive heterotopic ossification 2, while somatic mutations occur exclusively in diffuse intrinsic pontine glioma (DIPG, 32% of cases) and other malignancies 4. Clinically, momelotinib, a JAK1/JAK2 and ACVR1 inhibitor, was approved in 2023 for myelofibrosis treatment, demonstrating improvements in anemia, splenomegaly, and symptoms through ACVR1 inhibition 56.