HeLa cells were grown on a cover slip until ,50% confluence, and subsequently transfected with 1 mg of DNA for each of the fluorescent vectors analyzed, either alone or in combinations, by means of Effectene (Qiagen). Cells were incubated for 24 h, and fixed by incubating with 4% paraformaldehyde for 30 min at RT. Coverslips with fixed cells were mounted in Vectashield Mounting Medium (Vector Labs), and imaged on an OLYMPUS BX61 microscope.