DDC and nonpapillary renal cell carcinoma: The amino acid metabolism-related proteins, including SHMT1, BHMT, AHCY, ALDH1L1, DDC, AOX1, AFMID, KYNU, and HAAO, were downregulated in ccRCC compared to adjacent normal tissue, while NNMT was upregulated in ccRCC compared to adjacent normal tissue (Figure 1A).