IL-6 derived from breast cancer cells can activate STAT3 in MDSCs in 4T1 BALB/c mice, which stimulates the noncanonical NF-κB subunit p52 and causes RelB translocation into the nucleus, where they interact directly with the indoleamine 2,3-dioxygenase (IDO) promoter sequence to promote expression of IDO [30]. Here, IDO1 is linked to breast cancer.