Recently, Giallongo et al. identified a population of myeloid-derived suppressor cells (MDSC) in chronic myeloid leukemia (CML) patients that is part of the tumor clone and provides a leukemic friendly microenvironment mediated by ARG1, NOS2, reactive species of oxygen (ROS), cyclooxygenase 2 (COX2), TGFβ and immunosuppressive cytokine production, inhibition of NK function, and Treg expansion. Here, ARG1 is linked to chronic myelogenous leukemia, BCR-ABL1 positive.