The present invention relates to a novel HLF protein which is a member of the heregulin family. In particular, isolated nucleic acid molecules are provided encoding the human HLF protein. HLF polypeptides are also provided as are vectors, host cells and recombinant methods for producing the same. The invention further relates to screening methods for identifying agonists and antagonists of HLF activity. Also provided are diagnostic methods for detecting disorders of the regulation of cell growth and therapeutic methods for treating disorders of the regulation of cell growth.