In the last decade there have been four monogenic diseases for which seminal gene therapy trials have been conducted: ornithine transcarbamylase (OTC) deficiency, hemophilia caused by factor IX deficiency, severe combined immunodeficiency (SCID), and chronic granulomatous disease (CGD), the results of which are briefly summarized in Table 1 [5–12]. The gene discussed is OTC; the disease is hemophilia B.