However, there have also been important breakthroughs in the literature showing that secreted soluble ligands that bind to activating immune receptors do not always have an inhibitory effect, but, for example, in the case of the growth factor PD-L1, the ligand is able to inhibit the activity of NK cells, and in the case of the growth factor PDGF-DD to the NK cell receptor NKp44, it can also directly activate NK cells and thus induce the secretion of interferon gamma (IFN)-γ and tumor necrosis factor (TNF) by NK cells, which stop the growth of tumor cells [31,32]. The gene discussed is IFNG; the disease is neoplasm.