ARG1 and neoplasm: Interestingly, the hypoxia-inducible factors (HIF)-1α and HIF-2α are expressed differentially in M1- and M2-polarized macrophages [62] and regulate inducible NOS-2 (M1) and Arg-1 (M2), respectively (Figure 1), suggesting that tumor hypoxia may mediate the transition of M1 to M2.