PTEN and neoplasm: Regarding the possible mechanism of the interaction between IDO and PTEN, we might speculate that higher PTEN levels may originate from adjacent cells of the tumor microenvironment e.g., endothelial cells, T cells, etc. Based on a few recent studies, we might assume that IDO released from adjacent cells could trigger a PTEN expression loop (in TREG cells) through the decreased tryptophan level which could lead to cell death and apoptosis [23].