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. The gene discussed is UGT1A1; the disease is Crigler-Najjar syndrome.