IDO1 and head and neck squamous cell carcinoma: TDO2, the other rate-limiting enzyme of the KP (36), was constitutively expressed by all glioma samples, and expression was even higher in comparison to IDO1. Generally, expression status for TDO2 and kynurenine hydrolase (KYNU) was higher in GBM, while HNSCC expressed more kynurenine aminotransferases (KYAT) (Figure 1).