Proteins, downregulated in tumor tissues, were mainly involved in pathways related to kidney functions (collecting duct acid secretion, and proximal tubule bicarbonate reclamation), and PPAR signaling (e.g., ACOX2, FABP3, CPT2, EHHADH, RXRA), citrate cycle (e.g., CS, FH, IDH2, ACO2, SDHA), oxidative phosphorylation (e.g., ATPs, NDUFABs, NDs), biosynthesis of amino acids (e.g., DDC, AFMID, PSAT1, GLS), fatty acid degradation (e.g., HADH, EHHADH, ACAT1), anion transmembrane transport (e.g., SLC22A6, ABCC2, SLC22A7), and xenobiotic metabolic process (e.g., EPHX1, UGT1A9, BPHL) (Fig. 4c). Here, DDC is linked to neoplasm.