ARG1 and amyotrophic lateral sclerosis: To further characterize the phenotype of infiltrating macrophages in the peripheral nerves of mSOD1 ALS mice, immunostaining for arginase-1 (Arg-1), a marker of anti-inflammatory M2 macrophages, and inducible nitric oxide synthase (iNOS), a marker of pro-inflammatory M1 macrophages, was performed.