CD19 and cancer: Despite the unclear data landscape in preclinical models with regard to enhanced persistence of CAR T cells, most likely due to the short observation time of less than 3 months in most mouse studies [157,161], greater persistence in 4-1BB-costimulated CD19-CAR T compared to CD28-costimulated cells were observed in various clinical trials and are in general accepted, even though the value of CD19-CAR persistence remains elusive and seems to be cancer-specific [2,3,162].