More recently, Sumitomo et al. aimed at investigating the involvement of tryptophan 2,3-dioxygenase (TDO) and of indoleamine 2,3-dioxygenase 1 (IDO1) in both cancer development and resistance to ICIs in patients with mRCC or non-metastatic disease [29]. This evidence concerns the gene IDO1 and metastatic neoplasm.