HPR and colorectal carcinoma: In the training set, the concentrations of 17 amino acids including Gly, Ala, Val, Leu, Phe, Trp, Pro, Arg, Met, Tyr, Cyss, His, Kyn, Hia, Hpr, Sar, and Apa significantly decreased in CRC patients compared with that in HV (P < 0.05, student’s-t test), while Gln, Ser, Glu, Asn, Opr, Orn, Lys, Asp, Cys, Amp, and SDMA obviously increased in CRC (all P < 0.05, student’s-t test), and four amino acids Ile, Thr, Cit, and ADMA didn’t show any differences between CRC patients and HV (Fig. 1).