IDO1 and breast cancer: In breast cancer, immune suppressor cells, for example, myeloid-derived suppressor cells (MDSCs) and M2 macrophages, can release suppressive factors, such as IL-10, indoleamine dioxygenase 1 (IDO1), reactive oxygen species (ROS) and nitric oxide (NO), to suppress T and NK cell functions and promote tumour growth and metastasis [43, 44].