Possible mechanisms include the pre-exiting of antigen negative clones, such as CD34+CD19-CD22+ B-cell progenitors [97,113], gene mutation [116], splicing events [117], the transcriptional plasticity of leukemia cell [114], a reversible antigen loss through trogocytosis [118], the instant introduction of the CAR-T gene to the leukemia cell [119], and lineage switch-induced antigen loss [120]. The gene discussed is CD19; the disease is leukemia.