CD4 and acute lymphoblastic leukemia: Age at the time of the first ALL, the intensity of the treatment received prior to CAR T-cell therapy, including HSCT, the CART19 type/construct, the T-cell status/sub-type used to perform the CART therapy (CD8+, or CD4+, naïve or memory, etc.), and the persistence of the CAR T-cell may all play a role in this persistent defect.