ALDH18A1 and liver cancer: Proline metabolism has been confirmed to enhance the tumorigenesis in liver cancer as two enzymes corresponding to proline biosynthesis are upregulated (pyrroline-5-carboxylate reductase (PYCR1), aldehyde dehydrogenase 18 family member A1 (ALDH18A1)), and one proline catabolic enzyme is downregulated (proline dehydrogenase (PRODH)) (Ding et al., 2021).