IDO1 and acute myeloid leukemia: Subgroup analysis indicated that the gene expression level of IDO1 was lower in AML-M3 subtype according to French American British (FAB) classification criteria (also known as acute promyelocytic leukemia) (Fig. 1c) and 20–29 age group (Fig. 1d), while the > = 70 years age group showed relatively higher gene expression of IDO1 (Fig. 1d).