A recent study demonstrated that in a CLL patient who received CAR-T therapy, the structure of TET2 was disrupted when CAR lentivirus inserted into it, and consequently, CAR-T cells could proliferate capriciously without the inhibition of TET2. In this condition, more than 90% of CAR-T cells in this patient’s body have the same T cell receptor beta repertoire, which means they are from the same cell. The gene discussed is TET2; the disease is B-cell chronic lymphocytic leukemia.