PYCR2 and esophageal squamous cell carcinoma: The change of proline expression is the most significant factor in amino acid metabolism in hepatocellular carcinoma (HCC), and PYCR2 is abnormally expressed in the esophageal squamous cell carcinoma (ESCC), indicating that PYCR2 may play a key role in cancer progression (Tang et al., 2018; Sun et al., 2019).