SPR and neoplasm: By taking into account that Gch1 is a rate-limiting enzyme in BH4 production by Spr gene-encoded enzyme, that is, sepiapetrin reductase (45), it can be concluded that Gch1-Spr gene expression axis might be targeted in 96 h-SPS–treated tumor, but not in RPMI group, when the expression of Gch1 and Spr genes in these groups was compared with normal saline reference group.