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Ferroptosis is mainly caused by iron-mediated peroxidation of phospholipids and has recently attracted attention due to its involvement in various diseases. At the center of it is supposedly the inability of glutathione peroxidase 4 (GPX4) to reduce excess peroxidized phospholipids (e.g., phosphatidylcholine hydroperoxide (PCOOH)) that trigger ferroptosis. However, the involvement of enzymes other than GPX4 in ferroptosis is scarcely known. To elucidate this matter, we evaluated the uptake of PCOOH in a GPX4 knockout (KO) human hepatoma cell line HepG2 generated using CRISPR-Cas9. After confirming that GPX4 expression in the KO cells was below the detection limit, we cultured both wild-type (WT) and GPX4 KO HepG2 cells in a medium containing 50 μM PCOOH for 1-8 hours. By analyzing the level of PCOOH and its reduction product (phosphatidylcholine hydroxide, PCOH) in cells using liquid chromatographytandem mass spectrometry, we detected the cellular uptake of PCOOH. On top of this, we detected a large amount of PCOH not only in WT HepG2 but also in GPX4 KO HepG2, thus indicating the notable involvement of enzymes other than GPX4 (e.g., other GPX family, glutathione S-transferase, thioredoxin, or peroxiredoxin) in reducing PCOOH. Further corroboration of these findings hopefully leads to the development of novel methods to prevent ferroptosis-related diseases by targeting enzymes other than GPX4.