ACVR1B (activin A receptor type 1B) is a transmembrane serine/threonine kinase that functions as a type I receptor in the activin signaling pathway. As part of a heteroligand receptor complex with type II activin receptors (ACVR2A/2B), ACVR1B transduces activin signals by phosphorylating SMAD2/3 proteins, which then complex with SMAD4 and translocate to the nucleus to regulate gene transcription 1. The Activin-A-ACVR1B axis plays distinct roles across physiological contexts: it drives pathogenic Th17 cell differentiation through ERK phosphorylation during autoimmune neuroinflammation 2, and synergistically with TGF-β signaling regulates skeletal muscle fiber size, regeneration, and extracellular matrix deposition 3. Genetically, ACVR1B variants are associated with human muscle strength variation, with the rs2854464 A-allele conferring approximately 2% greater knee strength 4. Pathologically, ACVR1B loss accelerates development of pancreatic precancerous lesions (PanIN and IPMN) in conjunction with KRAS mutations in both acinar and ductal cells 5, and ACVR1B functions as a colon cancer-specific driver gene 6. ACVR1B expression is upregulated during sepsis and influences neutrophil responses 7. The Activin-A-ACVR1B interaction also regulates tumor cell apoptosis in pituitary neuroendocrine tumors 1.