The present invention relates generally to a biologically active complex comprising at least two heterologous molecules. More particularly, the biologically active complex of the present invention comprises at least two polypeptides or parts, fragments, truncates or protease-activated forms of one or more of the polypeptides wherein the complex alone or in association with a receptor, ligand or other molecule facilitates proliferation, differentiation and/or survival of a cell. The identification of the biologically active complex of the present invention permits the assay for agonists and antagonists of the formation of the biologically active complex as well as therapeutic and diagnostic reagents based on the biologically active complex or interaction between the biologically active complex and a receptor, ligand or other molecule.