CD19 and acute lymphoblastic leukemia: Moreover, antigen masking due to accidental transfection and the consequential aberrant expression of the anti-CD19 CAR-T cells by BCP-ALL cells [60]; the transfer of CD19 from blast to T cells via a phenomenon known as trogocytosis [61]; or a phenomena of leukemia plasticity with phenotype conversion, in particular, in patients harboring KMT2A gene rearrangements [62,63,64] are other modalities by which leukemic cells can modulate antigen expression.