GPX4 and hepatocellular carcinoma: Further, using 5 online phosphorylation prediction tools, we identified that the Ser2, Ser40, Ser45, and Ser112 sites were the unique and potential phosphorylation sites of GPX4 (Figure 4C), and only the Ser2 site mutation decreased the mitochondrial p-GPX4 level in HCC cells (Figure 4D), suggesting that the Ser2 site was one of the functional regulatory sites associated with GPX4 phosphorylation.