The capsid proteins of adenovirus serotype 5 (Ad5) are key to the virus' highly efficient cell binding and entry mechanism. In particular, the penton base plays a significant role in both viral internalization and endosome penetration. We have produced an adenovirus penton fusion protein (HerPBK10) containing moieties for DNA transport and targeted delivery to breast cancer cells. HerPBK10 binds DNA through a polylysine appendage, while the EGF-like domain of the heregulin-alpha(1) isoform is used as the targeting ligand. This ligand binds with high affinity to HER2/3 or HER2/4 heterodimers, which are overexpressed on certain aggressive breast cancers. In addition, this ligand is rapidly internalized after binding, thus adding to the utility of heregulin for targeting. HerPBK10 binds MDA-MB-453 breast cancer cells in a receptor-specific manner, and mediates the entry of a reporter plasmid in MDA-MB-453 cells in culture. Delivery can be competed by excess heregulin peptide, thus confirming receptor specificity. Importantly, the penton segment appears to contribute significantly to enhanced delivery. Complexes containing HerPBK10 and DNA have been optimized to provide targeted gene delivery to breast cancer cells in vitro. We demonstrate that delivery can be accomplished in the presence of serum, thus suggesting a potential use for in vivo delivery.