Unfortunately, in vivo a cationic DNA-containing particle will bind to the first soluble or extracellular matrix polyanionic protein it encounters. This will prevent most particles from reaching their target and will eventually release DNA from the complexes. Anionic proteoglycans are therefore too ubiquitous to serve as receptors for synthetic vectors. As discussed above, integrins share many functional properties with syndecans. Several integrins bind and internalize zwitterionic RGD peptide-presenting particles in vivo [16]. After the manner of adenovirus, RGD peptides were chemically conjugated to polycations such as polylysine [17,18] or polyethylenimine (PEI) [19] and complexed with DNA. The resulting particles were shown to deliver genes to epithelial cells in culture up to 100-fold better than the plain polycation /DNA complexes. A control experiment using RGE-coated particles confirmed the enhanced transfection to be due to αv β5 integrin-mediated cell entry.