After transfecting HEK-293T cells with Flag-GPX4, the cells were immunoprecipitated using an anti-FLAG antibody. the GPX4 proteins were isolated using sodium dodecyl-sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) on a 4-12% gradient gel (Invitrogen, Carlsbad, CA, USA) and stained with Coomassie blue. Mass spectrometric methylated peptide sequencing was performed on selected bands that were removed from the gel. Mass spectrometric data were processed by PEAKS studio X.