Liver-directed gene and cell therapies represent a promising therapeutic strategy for many genetic disorders. However, they are hampered by inefficiency. The problem of inefficiency could be overcome by providing a selective growth advantage to corrected cells to allow them to repopulate the liver. This work aimed to characterize a novel method for in vivo selective expansion of hepatocytes using the common medication acetaminophen (APAP). In the liver, APAP is metabolized by cytochrome P450 enzymes to form a hepatotoxic intermediate. Cytochrome P450 enzymes require a cofactor, cytochrome P450 reductase (Cypor), for activity. Therefore, knockout of Cypor prevents APAP toxicity. A rare population of Cypor-deficient cells can thus be selectively expanded in vivo by APAP treatment. This work describes the application of this strategy to both liver-directed viral gene therapy and hepatocyte transplantation.