Gene therapy is one of the most important aspects of molecular targeted therapeutic approaches for tumors. A small molecule targeting carrier plays an important role in this process. PI, a new peptide found in our phage library, has been specifically suggested, combined with the human triple-negative breast cancer cell line MDA-MB-231, and may be developed as a targeting/individualization therapy strategy to be applied in breast cancer research. In this study, we further investigated whether this peptide could carry exogenous protein to the target cells by forming a fusion peptide. PI-GST and PI-TK were cloned into plasmids and used for expression studies, analyses of PI-mediated protein delivery efficiency, and to investigations into the effect of PI on thymidine kinase/ganciclovir-mediated cytotoxicity. Biodistribution profiles were also investigated in vivo. The results showed the PI fusion protein was expressed correctly in vitro, and could carry GST into the target cells. Under certain conditions, PI-TK sensitizes cells to ganciclovir more efficiently than TK. In vivo there was a trend for increased inhibition of tumor growth with PI-TK when ganciclovir was present. Therefore, our results suggest the potential of PI as a new specific target carrier in breast cancer therapy.