IDO1 and neoplasm: In particular, IDO1 is overexpressed and constitutes a poor prognostic marker in many tumours including endometrial cancer (Ino et al., 2006), laryngeal squamous cell carcinoma (Ye et al., 2013), melanomas (Speeckaert et al., 2012), gastric cancer (Li et al., 2019), hepatocarcinoma (Pan et al., 2008), and cervical cancer (Inaba et al., 2010).