The present invention relates to a method for determining whether a compound is capable of binding to a BMP receptor kinase protein complex, the method comprising introducing a sample comprising the compound to the BMP receptor kinase protein complex and allowing the compound to bind to the BMP receptor kinase protein complex, wherein the BMP receptor kinase protein complex is comprised of a BMP type I receptor kinase protein and the BMP receptor kinase protein BRK-3. The invention further relates to a method for determining the concentration of a BMP receptor ligand in a clinical sample, the met-hod comprising introducing the sample comprising the ligand to a BMP receptor kinase protein complex and allowing the ligand to bind to the BMP receptor kinase protein complex, wherein the BMP receptor kinase protein complex is comprised of a BMP type I receptor kinase protein and BMP receptor kinase protein BRK-3. The invention further relates to a host cell co-transfected with an expression vector comprising a DNA sequence that codes for the BMP receptor kinase protein BRK-3 and an expression vector comprising a DNA sequence that codes for a BMP type I receptor kinase protein. The invention further relates to a host cell co-transfected with an expression vector comprising a DNA sequence that codes for a soluble or incomplete BMP type I receptor kinase protein and a soluble or incomplete BMP receptor kinsase protein BRK-3. The invention further relates to a method for determining whether a test compound produces a signal upon binding to a BMP receptor protein complex.