Polypeptides of the neuregulin (NRG) heparin binding domain (N-HBD) and nucleic acids coding therefor are disclosed. In particular, fusion polypeptides are produced that comprise, as a targeting structure, a N-HBD polypeptide, fragment, homologue or functional derivative and a protein to be targeted. This is fused to a polypeptide or peptide being targeted (P trg ) to cell surfaces rich in heparan sulfate proteoglycans to either activate or inhibit interactions at tyrosine kinase receptors. A preferred fusion polypeptide comprises an N-HBD, a spacer and the extracellular domain of erbB4, one of several receptors signaled by NRG, which is potent NRG antagonist. Such products are used to treat diseases or conditions where either agonism or antagonism at tyrosine kinase receptors has beneficial effects, including cancer and a multitude of diseases of the nervous system.