ARG1 and neoplasm: MDSCs, as a major immunosuppressive population in 4T1-bearing BALB/c mice (13, 20), contribute to tumor-immunosuppressive microenvironment not only by producing a serial of suppressive molecules such as Arg-1, NCF1, CYBB, TGF-β, and IL-10 (25, 26) but also by promoting recruitment and expansion of Treg cells via TGF-β and IL-10 (26).