The magnitude of the translational potential of epigenetic reprogramming in determining T-cell fate is highlighted in the case of a patient with chronic lymphocytic leukemia who achieved remission after CD19-CAR-T infusion, in whom inadvertent biallelic disruption in a single clone of the TET2 gene that regulates DNA demethylation occurred, leading to massive expansion of this clone, represented by 94% of circulating CAR-Ts [80]. The gene discussed is CD19; the disease is B-cell chronic lymphocytic leukemia.