CD19 and acute lymphoblastic leukemia: This has been demonstrated by a single-cell RNA sequencing (scRNA-seq) study of an ALL-B patient who relapsed with CD19-negative B cell ALL after anti-CD19 CAR T-cell therapy, in which it was shown that CD19-negative leukemic cells were present prior to CAR T-cell therapy, which would have caused the CD19-negative relapse by simple immune pressure (27).