Although the non-MHC-restricted tumor-killing activity of CIK cells is mainly mediated by the interaction of NKG2D with stress-inducible molecules (i.e., MIC A/B and ULBPs) on target cells [39–41], the pro-inflammatory cytokines secreted by CIK cells also play a role in killing cancer cells. Here, KLRK1 is linked to neoplasm.