Given that IDO1 and TDO are often expressed in malignant tumors and cause unwanted suppression of the immune system via their production of Kyn (27, 61), we speculate that a low level of NO might actually be boosting the IDO1 and TDO heme contents, thereby increasing Kyn production and the resultant immune suppression that helps tumor cells escape from immune surveillance. Here, TDO2 is linked to cancer.