We examined the immunohistochemical expression of two markers of microglial phenotype, the arginine-metabolizing enzymes inducible nitric oxide synthase (iNOS) and arginase1 (Arg1), in the spinal cord of a mouse model carrying an ALS-linked mutant human superoxide dismutase transgene (SOD1G93A) and in non-transgenic wild-type (WT) mice. Here, ARG1 is linked to amyotrophic lateral sclerosis.