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