Since the creation of CARs in 1993, the survival of engineered T cells has posed a serious obstacle to successful clinical outcomes. As a result, several generations of CARs have been introduced, each with its own design and attributes that prolong cell survival . The first generation was composed of a single-chain variable fragment (scFv) and a cytoplasmic CD3ζ signaling domain (Fig. ) . However, it was unsuccessful in activating the T cells through the CD3ζ domain. First-generation CARs cannot produce a sustained anti-tumor response or cytokine release due to poor signaling . To strengthen signal transduction, the second generation of CARs included a costimulatory domain, such as CD28 or 4-1BB, to promote T cell activation, survival, and proliferation of the engineered cells (Fig. ) . Kymriah and Yescarta, both FDA-approved therapies for B-ALL and Large B-cell lymphoma (LBCL), are second-generation CAR-T medications. Second-gen CARs demonstrate the importance of including a costimulatory domain to achieve stronger persistence and overall function of CAR-T cells. A clinical trial administered CAR-T cells containing 4-1BBζ to 14 patients with chronic lymphocytic leukemia, finding that the cells persisted for over 4 years among patients who achieved complete remission . Another use involving the 4-1BB costimulatory receptor promoted the development of the T CM subset with enhanced metabolic efficiency through FAO and OXPHOS pathways . Additionally, autologous T cells expressing a CD28ζ domain yielded T SCM and T CM cells with increased glycolytic metabolism. Overall, CARs incorporating 4-1BBζ or CD28ζ tend to promote the differentiation of T SCM and T CM cells, respectively, leading to enhanced persistence. The important idea from the second generation is that including a costimulatory domain is crucial for substantive T cell activation, ultimately leading to improved anti-tumor response.