Activin receptor type-1B (ACVR1B) is also known as Activin receptor-like kinase 4 (ALK-4), Serine/threonine-protein kinase receptor R2 (SKR2), which belongs to the protein kinase superfamily, TKL Ser/Thr protein kinase family and TGFB receptor subfamily. ACVR1B contains one GS domain and one protein kinase domain. The catalytic activity of ACVR1B is ¡°ATP + [receptor-protein] = ADP + [receptor-protein] phosphate¡±. ACVR1B acts as a transducer of activin or activin like ligands (e.g., inhibin) signals. Activin binds to either ACVR2A or ACVR2B and then forms a complex with ACVR1B. These go on to recruit the R-SMADs SMAD2 or SMAD3. ACVR1B also transduces signals of nodal, GDF-1, and Vg1.