IDO1 and renal cell carcinoma: Moreover, considering RCC and HNSCC separately, there is no correlation between kyn/trp ratio and response, suggesting that in these two types of tumors, the IDO activity could have a marginal role in the complex mechanism determining the primary resistance to immunotherapy or concurrent medications, able to induce TDO expression like the steroids, and nutritional state and infection may have a confounding effects on results.