No statistically significant correlations were observed between tumor diameter and tryptophan, kynurenic acid, indoleamine-2,3-dioxygenase (IDO), kynureninase, kynurenine aminotransferase, 3-hydroxykynureninase, anthranilic acid, quinolinic acid, or picolinic acid. This evidence concerns the gene IDO2 and neoplasm.