ARG1 and neoplasm: To further explore whether the reduced Gr1+ and Arg1+ cells visualized by IHC were a result of MEKi treatment and not induced by synergistic or chemotherapy-dependent effects, we treated BCM-2277 tumor–bearing mice daily with or without MEKi for 7 days, without any chemotherapy, before flow cytometry analysis of CD45+CD11b+ cells for Ly6G or Ly6C expression.