A recent study from Argentina of patients with relapsing–remitting MS (mean duration of disease of 5.2 years) suggested that circulating blood cells of MS patients expressed reduced levels of IDO1 and ARG1.14 This would provide a proinflammatory environment with increased availability of Trp and Arg for the Th1‐ and Th17‐driven autoimmune responses associated with MS pathogenesis.15 In the study reported here, the focus was on expression of IDO and ARG during the MS disease course. Here, ARG1 is linked to myeloid sarcoma.