Further, accumulation of extracellular adenosine [9], accelerated potassium flux derived from necrosis of cancer cells [10], and tumor-released kynurenine are mediated by enzyme indoleamine 2.3-dioxygenase (IDO) as a byproduct of tryptophan metabolism, which all collectively impair effector T cell function and survival. Here, IDO1 is linked to cancer.