CD8A and neoplasm: The enzyme co-factor tetrahydrobiopterin (BH4) can rescue the inhibition of kynurenine on T cells, upregulating the expression of GTP cyclohydrolase 1 (GCH1) and sepiapterin reductase-key enzymes in the BH4 synthesis process-significantly enhancing CD8+ T cell anti-tumor activity and controlling tumor growth in mouse bearing breast tumor [304].