In certain aspects, the disclosure provides soluble heteromeric polypeptide complexes comprising an extracellular domain of a type I serine/threonine kinase receptor of the TGF-beta family and an extracellular domain of a type II serine/threonine kinase receptor of the TGF-beta family. In some embodiments, the disclosure provides soluble polypeptide complexes comprising an extracellular domain of a type II receptor selected from: ActRIIA, ActRIIB, TGFBRII, BMPRII, and MISRII. In some embodiments, the disclosure provides soluble polypeptide complexes comprising an extracellular domain of a type I receptor selected from: ALK1, ALK2, ALK3, ALK4, ALK5, ALK6, and ALK7. Optionally the soluble complex is a heterodimer. In certain aspects, such soluble polypeptide complexes may be used to regulate (promote or inhibit) growth of tissues or cells including, for example, muscle, bone, cartilage, fat, neural tissue, tumors, cancerous cells, and/or cells of hematopoietic lineages, including red blood cells. In certain aspects, such soluble polypeptide complexes are can be used to improve muscle formation, bone formation, hematopoiesis, metabolic parameters, and disorders associated with these tissues, cellular networks, and endocrine systems.Dans certains aspects, l'invention concerne des complexes de polypeptides hétéromères solubles comprenant un domaine extracellulaire d'un récepteur à sérine/thréonine kinase type I de la famille de TGF-bêta et un domaine extracellulaire d'un récepteur à sérine/thréonine kinase type II de la famille de TGF-bêta. Dans certains modes de réalisation, l'invention concerne des complexes de polypeptides solubles comprenant un domaine extracellulaire d'un récepteur de type II choisi parmi : ActRIIA, ActRIIB, TGFBRII, BMPRII et MISRII. Dans certains modes de réalisation, l'invention concerne des complexes polypeptidiques solubles comprenant un domaine extracellulaire d'un récepteur de type I choisi parmi : ALK1, ALK.2, ALK3, ALK4, ALK5, ALK.6 et ALK
