Antigen escape, or loss of antigen density, has been recognized as a mechanism underlying tumor relapse in patients treated with CAR T cells directed at a variety of antigens, including CD19 (B cell malignancies), B cell maturation antigen (BCMA (multiple myeloma)), and epidermal growth factor receptor variant III (EGFRvIII (glioblastoma multiforme)) [160,189,190]. Here, CD19 is linked to plasma cell myeloma.