Herein, apart from tryptophan-KP metabolite variations, we also observed increase of tryptophan-neurotransmitter metabolites, such as 5-hydroxy-L-tryptophan, serotonin, and tryptamine, in RCC (Figure 4), indicating neuroendocrine involvement of tryptophan metabolism in human RCC (27). This evidence concerns the gene NPPA and renal cell carcinoma.