Targeted gene transfer by nonviral vectors can be achieved through incorporation of specific ligand( s ) into the vectors. In this study, the effects of incorporation of an anti -ErbB2 single -chain antibody fragment ( ScFv ) into nonviral vectors for targeted gene delivery were investigated. The ML39 ScFv, selected from a human ScFv phage display library and affinity matured in vitro ( K d =1Â10 À 9 M ), was used as ligand specific for the extracellular domain of the tumor surface protein, ErbB2. Two approaches were taken: ( a ) development of a vector that is composed of a bifunctional fusion protein capable of binding DNA with the ErbB2 -specific ML39 ScFv at its N -terminus and a truncated form of human protamine at its C -terminus, and ( b ) formulation and evaluation of delivery vectors consisting of three independent components including ML39 ScFv, protamine, and cationic lipids. We demonstrate that fusion proteins comprised of the ML39 ScFv and a truncated form of protamine, denoted as ScFv -P -S, can selectively deliver exogenous DNA into ErbB2( + ) cells, with an 8 -to 10 -fold increase in expression levels of the luciferase reporter gene in ErbB2( + ) cells as compared to ErbB2( À ) cells. In addition, vectors formulated by appropriately mixing DNA, ScFv, protamine, and lipids in vitro could even more efficiently deliver the reporter gene into ErbB2( + ) cells with approximately 5 -fold increase in gene expression in ErbB2( + ) cell as compared to ErbB2( À ) cells. Expression and refolding of the ScFv fusion proteins, in addition to determination of optimal conditions for vector development using these approaches, are discussed.