Normalized luciferase expression (expressed as relative light units per mg of total cell protein) in COS-7 cells treated with polyplexes formed using ester-functionalized polymers 1a-e or LPEI. Cells were treated with polyplexes in serum-free media for four hours, followed by incubation in serum-containing media for 48 hours. These experiments were conducted in replicates of three using complexes formed at ten different DNA/polymer ratios (w/w) ranging from 1:1 to 1:10. Values of normalized luciferase expression shown correspond to the DNA/polymer ratio determined to mediate the highest levels of transfection for each polymer and correspond to ratios of 1:1 (for polymers 1a, 1c, 1e, and LPEI), 1:9 (for polymer 1b), and 1:7 (for polymer 1d) and are reported as averages with standard deviation. Figure S1 of the Supporting Information shows a plot of these data using a log scale on the y-axis. Results are presented as an average of three experiments ± standard deviation. Results marked with an asterisk (*) indicate results determined to be statistically different from all others (p<0.05) using Student's t-test.