CD8A and cancer: Given that optimal costimulatory domains may be different for CD4- and CD8-CAR T cells, CD4-CAR T cells can directly kill cancer cells, and single vectors can be used to make 3rd generation CARs for enhancing persistence of both CD4- and CD8-CAR T cells, future studies focused on optimizing costimulation for both CD4- and CD8-CAR T cells should benefit patients with solid tumors.