Novel 2 polypeptides with binding affinity for the p185^HER2 receptor, designated heregulin 2- and heregulin 2-, have been identified and purified from human tissue. The cDNA encoding the novel heregulin 2- has been isolated from human tissue and sequenced. Provided herein is nucleic acid sequence of the heregulin 2- useful in the production of heregulin 2- by recombinant means. Further provided an amino acid sequence of heregulin 2- and heregulin 2-. Heregulins and their antibodies are useful as therapeutic agents and in diagnostic methods.