IDO1 and neoplasm: Although CD11b+ myeloid cells have been reported to inducibly express IDO1 in the tumor microenvironment,27 interestingly, double staining for IDO1 and CD11b revealed that CD11b+ cells in the MLNs of l-1MTrp+CPA-treated mice generally did not express IDO1 (figure 4D), indicating that IDO1 induction primarily occurred in CD11b-negative cells.