Of greater relevance to the cancer therapy field, is that clear associations between high IDO1 expression and poor patient outcome have been documented for many difficult-to-treat cancers that include glioma [11], melanoma [12], prostate [13], endometrial [14], ovarian [15], breast [16], pancreatic [17], lung [18,19], and colorectal [20,21] cancers. Here, IDO1 is linked to cancer.