Abstract: Disclosed are (1) nucleic acid sequences, amino acid sequences, homologies, structural features and various other data characterizing a morphogen cell surface receptors particularly OP-1-binding cell surface receptors ALK-2, ALK-3, and ALK-6; (2) methods for producing receptor proteins, including fragments thereof, using recombinant DNA technology; (3) methods for identifying novel morphogen receptors and their encoding DNAS; (4) methods and compositions for identifying compounds capable of modulating endogenous morphogen receptor levels; and (5) methods and compositions for identifying morphogen receptor binding analogs useful in the design of morphogen agonists and antagonists for therapeutic, diagnostic and experimental uses.