IDO expression by AML cells. IDO gene expression by AML blasts has been analyzed using a quantitative RT-PCR method in 54 newly diagnosed patients. IDO mRNA level was compared with that of Abelson (ABL) control gene. According to the IDO transcript level, AML patients could be subdivided into 2 groups. In particular, a group of patients (n = 40) expressed IDO at the same level as normal BM-derived CD34⁺ cells (IDO⁻). The remaining samples (n = 14) showed a significantly increased IDO mRNA expression that was at a level comparable with that of the positive control placenta cells (IDO⁺).