As summarized in the elegant review on the human liver-directed genome editing by Ginn et al. [7], already in the late 1990’s, clinical studies were executed with the aim to correct ornithine transcarbamylase deficiency—the most common genetic urea cycle disorder in humans—through adenovirus-mediated, hepatocyte-directed expression of a functional ornithine transcarbamylase gene variant. The gene discussed is OTC; the disease is hyperinsulinemic hypoglycemia, familial, 4.