Doxorubicin (Dox)-resistant clones of human colon (HT29), lung (A549), and leukemia (K562) cell lines, a murine chemoresistant breast cancer cell line (JC), and human metastatic mesothelioma (HMM) were shown to express high levels of IDO1 and displayed enhanced subcutaneous growth in vivo relative to chemosensitive parental cell lines. This evidence concerns the gene IDO1 and breast carcinoma.