Three types of chimeric mice with a humanized liver are frequently used for studies: urokinase-type plasminogen activator/severe combined immunodeficiency (uPA/SCID) mice, NOG mice expressing a thymidine kinase transgene (TK-NOG mice), and Fah−/−/Rag2−/−/Il-2rg−/− (FRG) mice (humanized liver FRG mice) [124–126]. The gene discussed is PLAU; the disease is severe combined immunodeficiency.