To explore the potential of IDO1 as a therapeutic target for CLL, we treated mice after adoptive transfer of Eμ-TCL1 leukemia cells with the IDO1 modulator 1-methyl-D-tryptophan (1-MT) which resulted in a minor reduction in leukemia development which disappeared over time. This evidence concerns the gene IDO1 and B-cell chronic lymphocytic leukemia.