The largest barriers to gene delivery are restricted cytoplasmic diffusion and the nuclear membrane [13,40]. DNA particles entering fast dividing cells in culture are less confronted by these problems. Transfection of synchronized cells showed mitosis to be a key event for transfection to occur [41,42]. It may therefore just happen that DNA complexes (or free DNA not yet degraded) sitting immobile in the cytoplasm are dragged along with chromosomes passing nearby and become incidentally sequestered in the nuclei of the daughter cells during telophase. This would explain why most transfected cells appear as doublets (Fig. 6).