PML arising post‐CAR‐T is likely to be a cumulative effect of multiple immunosuppressive insults, including lymphodepletion with fludarabine [13], corticosteroids for CAR‐T immunotoxicity [14], post‐CAR CD4+ lymphopenia [2], and B‐cell aplasia/hypogammaglobulinaemia. Here, CD4 is linked to immune system toxicity.