CD19 and acute lymphoblastic leukemia: Of the known escape mechanisms, the best defined etiology of disease relapse has been due to target antigen loss, and recent clinical data indicated that 7–33% of responders in CAR-19-T cell trials for B-ALL have relapsed due to loss of cell-surface CD19 (12, 16), which supports the immunoediting hypothesis proposed by Schreiber and colleagues in 2002 (17).