In tumor microenvironments, MSCs can be activated by pro-inflammatory cytokines IFN-γ, TNF-α, or IL-1β [52,69,83,84,85], which are secreted by tumor cells and macrophages and then produce immunomodulatory molecules, such as IDO, PGE2, IL-6, IL-10, HLA-G5, and nitric oxide (NO) (reviewed in [86]). This evidence concerns the gene IDO1 and neoplasm.