It has been demonstrated that tryptophan degradation is induced by IFN-γ in a number of cells and cell lines (SK-N-SH, neuroblastoma; T 24, J 82, bladder carcinoma; A 431, epidermoid carcinoma; normal dermal fibroblasts, U 138 MG, glioblastoma; SK-HEP-1, hepatoma; A 549, lung carcinoma; A 498, kidney carcinoma) [21]. Here, IFNG is linked to squamous cell carcinoma.