The interactions between NK cells and the tumor microenvironment include: (1) soluble factor production (transforming growth factor-beta, indoleamine 2,3-dioxygenase, interleukin (IL)-4, prostaglandin E2) by tumor cells or other cells in the tumor microenvironment (Castriconi et al., 2003; Marcenaro et al., 2005); (2) inhibitory cells at the tumor site, which include myeloid-derived suppressor cells, regulatory T cells, tumor-associated macrophages, tumor-associated fibroblasts, and tumor cells (Pietra et al., 2016); and (3) dysfunctional NK cells at the tumor site. Here, IDO2 is linked to neoplasm.