In this paper, we present the role of autologous and allogeneic monocytes from healthy individuals and those of the cancer patients, with a number of distinct cancers, in activating the function of natural killer (NK) cells, in particular, in induction of IFN-γ secretion by the NK cells and the functional capability of secreted IFN-γ in driving differentiation of the tumor cells. This evidence concerns the gene IFNG and cancer.