ARG1 (Arginase 1), is a marker for the M2 anti-inflammatory macrophage subset that hydrolyzes l-arginine into urea and ornithine, and plays an important role in wound healing, antiviral immune responses, and atherosclerosis. The inherited deficiency of this enzyme results in argininemia, which is an autosomal recessive disorder characterized by hyperammonemia. Recent studies show that intracellular pathogens induce the expression of ARG1 in mouse macrophages through the Toll-like receptor (TLR) pathway. The deletion of this enzyme in macrophages favors host survival. Other studies indicate that ARG1 also plays a role in T cell responses and selective antagonists of ARG1 might prove beneficial in fighting diseases in which T-cell responses are inappropriately suppressed.