Chimeric Antigen Receptors. CARs target surface antigens in an MHC-independent fashion and consist of an ectodomain, hinge domain, transmembrane domain, and endodomain. The initial trials tested first-generation CARs that have a single cytoplasmic domain. Current trials are testing second- and third-generation CARs that have combinations of signaling domains. (Blood. 2014;123:2625-2635.)