CD5 and acute lymphoblastic leukemia: One preclinical study of a CD5-CAR T-cell with CD28 costimulatory domain led to surface downregulation of CD5 in CAR T-cells with minimal fratricide, good expansion, and significant in vitro cytotoxicity against T-ALL cell lines [30]; however, with the use of 4-1BB as the costimulatory domain, the CD5-CAR construct demonstrated more fratricide [31].