We found that IL-15-expressing CAR-T cells display better in vitro expansion, possibly due to better retained viability and a “younger” phenotype with a greater fraction of TCM cells, which translates into better in vitro recursive killing activity and, importantly, more potent in vivo antitumor efficacy in two murine tumor models with an underlying mechanism involving the improved in vivo expansion and tumor infiltration of those cells. This evidence concerns the gene IL15 and neoplasm.