In addition to IL-6, which can function as a molecular switch between iTREG and TH17 cell differentiation, as described above, Sharma et al. [100] identified that indoleamine 2,3-dioxygenase (IDO) [101], a tryptophan-catabolizing enzyme produced by plasmacytoid dendritic cells (pDCs) and potentially other cells, maintains the TREG/TH17 balance in tumour-draining lymph nodes by regulating IL-6 production. This evidence concerns the gene IDO1 and neoplasm.