To our knowledge, this first-in-human, disease burden-guided study evaluated the feasibility, safety, and efficacy of donor-derived allogeneic interleukin-15-activated cytokine-induced killer cells (IL15-CIK) combining T-cell and natural killer cell properties for post-transplant disease control.<h4>Methods</h4>In a prospective, multicenter phase I/II trial (EudraCT 2013-005446-11) and an identically designed pilot study, 53 adult and pediatric patients with HR leukemia received 56 courses of IL15-CIK monotherapy after human leukocyte antigen (HLA)-matched or HLA-mismatched transplantation. The gene discussed is IL15; the disease is leukemia.