Gene transfer can be used to engineer T cells to express CARs that target antigens in an MHC-independent manner. CARs are fusion proteins composed of an extracellular portion that is usually derived from an antibody and intracellular signaling modules derived from T cell signaling proteins. First-generation CARs contain CD3ζ, whereas second-generation CARs possess a costimulatory endodomain (e.g. CD28 or 4-1BB) fused to CD3ζ. Third-generation CARs consist of two costimulatory endodomains linked to CD3ζ. Abbreviations: CAR, chimeric antigen receptor; MHC, major histocompatibility complex; scFv, single-chain variable fragment; TCR, T-cell receptor.