CD19 and leukemia: A novel antigen escape mechanism reported in ALL entails class switching to an acquired myeloid phenotypic leukemia, as seen in two patients with MLL-rearranged ALL treated with anti-CD19-directed CAR T cell therapy [192]; in principle, similar class switching phenomena could occur in such leukemias when treated with potent CAR-modified IECs directed at myeloid antigens.