In vivo, proCAR-4 T cells displayed a level of MC38-HER2 tumor control that was indistinguishable from the CD28TM reference CAR-T cells (Figure 6e, Figure 6—figure supplement 2), thereby closing the functional gap that was apparent between proCAR-3 and the CD28TM CAR in the previous experiment. Here, ERBB2 is linked to neoplasm.