Abstract: The present invention relates to a method for determining whether a compound is capable of binding to a BMP receptor kinase protein complex. The invention further relates to a method for determining the concentration of a BMP receptor ligand in a clinical sample. The invention also 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 kinase 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.