CD19 and neoplasm: Grupp et al. (27) performed a flow cytometry test on a pediatric case of CD19-negative relapse following CAR T-cell therapy and found that the small number of pre-existing CD19-negative clones in tumor cells prior to CAR T-cell therapy were transformed to dominant clones under the selective therapeutic stress of CD19 CARs, thus resulting in a CD19-negative relapse.