The present invention is directed to a method of in vivo and ex vivo gene delivery, for a variety of cells. More specifically, it relates to a novel carrier system and method for targeted delivery of nucleic acids to mammalian cells. More specifically, the present invention relates to carrier system comprising single-chain polypeptide binding molecules having an a region rich in basic amino acid and having the three dimensional folding and, thus, the binding ability and specificity, of the variable region of an antibody. The basic amino acid rich region can comprise oligo-lysine, oligo-arginine or combinations thereof. Such preparations of modified single chain polypeptide binding molecules also have ability to bind nucleic acids at the region rich in basic amino acid residues. These properties of the modified single chain polypeptide binding molecules make them very useful in a variety of therapeutic applications including gene therapy. The invention also relates to multivalent antigen-binding molecules having regions rich in basic amino acids. Compositions of, genetic constructions for, methods of use, and methods for producing basic amino acid tailed antigen-binding proteins are disclosed.