PEI enhances transfection of weakly anchoring cells. Transfection yields were determined by β-galactosidase assays and normalized to the yields obtained with PEI coating of the culture dish (to which it was assigned an arbitrary value of 100.0%). PEI and other attachment factors enhanced the transfection of loosely attaching cell lines such as PC-12 cells (graph A) and HEK-293 cells (graph B). No significant effect was observed with tightly attaching cells such as MYS fibroblasts (graph C). The bars show the average ± standard deviation of triplicate assays, which are representative of at least two independent experiments (* significantly higher than untreated control, p < 0.01).