CAR T-cell therapy is the first form of gene transfer therapy to gain FDA approval, with two approved therapeutics, Tisagenlecleucel (Kymriah) and axicabtagene ciloleucel (Yescarta or Axi-Cel) for CD19-positive refractory pre-B cell acute lymphoblastic leukemia and diffuse large B-cell lymphoma, respectively [43,48]. The gene discussed is CD19; the disease is B-cell acute lymphoblastic leukemia.