Besides, IDO1 mRNA transcripts are upregulated in lung tissues (36) and the serum KYN : Try ratio (KTR), an indicative of IDO activity, is greater in LC patients than healthy subjects (37), supporting the notion that higher KTR is associated with increased risk for LC (38), especially for LSCC in heavy smokers, because AhR (aryl hydrocarbon receptor) activates the carcinogenesis pathway of benzo(a)pyrene (BaP), a strong lung carcinogen derived from tobacco smoking (39). Here, AHR is linked to laryngotracheoesophageal cleft.