We have successfully used mouse models of specific human liver diseases to determine the MED and toxicity of vectors.1, 2 Two UGT1 knockout (KO) mouse strains are phenotypically similar to patients with Crigler-Najjar syndrome.3, 4 One strain contains the same point mutation as the Gunn rat, and the other has a neomycin cassette inserted into the Ugt1 gene locus.3, 4, 5, 6 Both UGT1 KO mouse models display lethal hyperbilirubinemia in the immediate postnatal period, and mice do not survive past day 11 of life without intervention. Here, UGT1A1 is linked to liver disorder.