IL-15 or IL-21, which also enhance NK cell functions, might prove to be more effective than IL-2 [e.g., IL-2 induces activation-induced cell death (AICD) of cytotoxic lymphocytes and expands suppressive T regulatory cells (Tregs)] (Waldmann et al., 2001; Miller et al., 2005; Barao et al., 2011; Denman et al., 2012; Josefowicz et al., 2012) in ensuring in vivo persistence of transferred functional NK cells for long-term control of leukemia (and of other cancers). This evidence concerns the gene IL2 and leukemia.