A major obstacle to developing small interfering RNAs (siRNAs) as cancer drugs is their intracellular delivery to disseminated cancer cells.Fusion proteins of single-chain fragmented antibodies (ScFvs) and positively charged peptides deliver siRNAs into specific target cells.However, the therapeutic potential of ScFv-mediated siRNA delivery has not been evaluated in cancer.Here, we tested whether Polo-like kinase 1 (PLK1) siRNAs complexed with a Her2-ScFvprotamine peptide fusion protein (F5-P) could suppress Her2+ breast cancer cell lines and primary human cancers in orthotopic breast cancer models.PLK1-siRNAs transferred by F5-P inhibited target gene expression, reduced proliferation, and induced apoptosis of Her2+ breast cancer cell lines and primary human cancer cells in vitrowithout triggering an interferon response.Intravenously injected F5-P/PLK1-siRNA complexes concentrated in orthotopic Her2+ breast cancer xenografts and persisted for at least 72 hours, leading to suppressed PLK1 gene expression and tumor cell apoptosis.The intravenously injected siRNA complexes retarded Her2+ breast tumor growth, reduced metastasis, and prolonged survival without evident toxicity.F5-P-mediated delivery of a cocktail of PLK1, CCND1, and AKT siRNAs was more effective than an equivalent dose of PLK1-siRNAs alone.These data suggest that F5-P could be used to deliver siRNAs to treat Her2+ breast cancer.