As immune escape mediated by downregulation or any functional antigen loss is the major cause of relapse in CD19-CAR-T cell-treated patients, it appears favorable for CD19-CAR-T cells to also engage with CD19low-expressing cancer cells [2,78,121] at the price of on-target off-tumor toxicity on healthy cells, e.g., neurons with low CD19 expression. This evidence concerns the gene CD19 and cancer.