(a) Relative luciferase activity values (normalized to controls) are shown for 1,400 lipidoids. ~7% of the library induced >50% gene silencing (shown in red). s.d. (not depicted for the sake of clarity) averaged 0.05 (n=4). (b) The tail length, (c) tail substitution number and (d) alkyl-amine composition influenced in vitro activity. Structural features that were common among efficacious materials had positive Relative Hit Rate values.