Some of the mechanisms elaborated by tumors that have been observed in ovarian cancer include downregulation of class I MHC molecules [135], upregulation of surface molecules that induce T cell anergy of exhaustion (i.e., PD-L1 [136]), releasing immunosuppressive molecules such as IDO [12], or Treg recruitment to the TME [9]. This evidence concerns the gene IDO1 and ovarian cancer.