Recent advances in genomic sequencing allow a new paradigm in hormonal research, and a comparative genomic approach facilitates the identification of receptors and signalling mechanisms for orphan ligands of the transforming growth factor beta (TGFbeta) superfamily. Instead of purifying growth differentiation factor 9 (GDF9) receptor proteins for identification, we hypothesized that GDF9, like other ligands in the TGFbeta family, activates type II and type I serine/threonine kinase receptors. Because searches of the human genome for genes with sequence homology to known serine/threonine kinase receptors failed to reveal uncharacterized receptor genes, GDF9 likely interacts with the known type II and type I activin receptor-like kinase (ALK) receptors in granulosa cells. We found that co-treatment with the bone morphogenetic protein (BMP) type II receptor (BMPRII) ectodomain blocks GDF9 activity. Likewise, in a GDF9-non-responsive cell line, overexpression of ALK5, but none of the other six type I receptors, conferred GDF9 responsiveness. The roles of BMPRII and ALK5 as receptors for GDF9 were validated in granulosa cells using gene "knock-down" approaches. Furthermore, we demonstrated the roles of BMPRII, ALK3 and ALK6 as the receptors for the orphan ligands GDF6, GDF7 and BMP10. Thus, evolutionary tracing of polypeptide ligands, receptors and downstream signalling molecules in their respective 'subgenomes' facilitates a new approach for hormonal research.