As higher IDO and ARG mRNA levels in CIS and MS patients may be because of a greater percentage of a major cell subset in patient PBMCs, correlations were sought between the relative mRNA expression levels of IDO and ARG and the frequency of CD3+CD4+ and CD3+CD8+ T cells, CD19+CD20+ B cells, CD3−CD56+ NK cells and CD14++ monocytes in PBMCs from HCs and CIS and MS patients. The gene discussed is IDO1; the disease is myeloid sarcoma.