CD19 and leukemia: One of the challenges is that BCMA can be downregulated or lost, causing resistance to this treatment; thus, novel CAR-T cells and bi-specific CAR-T cells need to be developed for effective therapy of multiple myeloma similarly to bispecific CD19–CD22, CD19–CD20, CD19–CD37, and other state-of-the art CAR-T cells developed against leukemia [23,24,25,26,27,28].