Arginase 1 (Arg1), an enzyme involved in metabolism of L-arginine and a marker for tumor-associated G-MDSC (20), and alternative “M2” macrophages (41), was produced at high levels by splenic Ly6G+ cells but not by Ly6Chi cells, further consistent with a role of Ly6Chi monocytes in pathology (Supplementary Figure 1A). This evidence concerns the gene ARG1 and neoplasm.