IDO1 and plasma cell myeloma: We have previously shown that microenvironmental levels of hepatocyte growth factor (HGF), a pro-angiogenic and immune-modulating cytokine, correlate with the expression of functional IDO1 in a subset of patients with multiple myeloma, a plasma cell malignancy that accounts for approximately 13% of hematological cancers in adults and has a unique ability to subvert and/or escape the anti-tumor immune response [19, 21].