ARG1 and neoplasm: It is appreciated that the conventional M1/M2 classification has limitations to capture the status of TAMs; however, arginase-1 (Arg1), an enzyme that metabolically dampens T-cell responses by deprivation of arginine, has been recognized as one of the key hallmarks of M2-skewed macrophages.4 Thus, Katzenelenbogen et al. performed in-depth profiling of Arg1+ tumor-infiltrating myeloid cells in MCA205 fibrosarcoma tissues.