IDO1 and breast cancer: In a study using the 4T1.2 murine mammary tumor model (34), PA alone or in combination with ER induced global changes in the gene expression profile within the TME, featured by a downregulation of chemokines important in MDSC and Treg recruitment (CCL5, CCL20, CCL22), immune checkpoint (programmed cell death protein 1, PD-1) and other inhibitory molecules (indoleamine 2,3-dioxygenase, IDO).