(a) Of the 96 LNPs tested for siRNA delivery to hepatocytes in mice, 68 had three or more tails (blue), 44 had a tertiary amine present in the original alkyl-amine (red) and 26 had an O13 tail length (yellow). Eighty eight percent of the LNPs exhibiting all three structural ‘efficacy criteria’ (overlap of circles) achieved nearly complete FVII knockdown. The probability of identifying efficacious LNPs decreased precipitously when any of the criteria were not met. (b) pKa values also significantly influenced delivery efficacy to hepatocytes in vivo. All lipidoid nanoparticles capable of mediating nearly complete Factor VII gene silencing had pKa values ≥5.5. Error bars represent s.d. (n=3).