MMP9 and neoplasm: COD is a flavoprotein oxidoreductase with good catalytic activity for cholesterol and can oxidize cholesterol to 4‐cholestene‐3‐one and H2O2.[13] 4‐Cholestene‐3‐one has certain antitumor efficacy and can promote the release of HMGB1 from the nucleus to the cytoplasm of tumor cells, block the nuclear translocation of HIF‐1α, and inhibit the activation of MMP‐2 and MMP‐9, thus inhibiting the invasion and metastasis of tumors.[14] Additionally, H2O2, another cholesterol decomposition product catalyzed by COD, can react with hemin in the Fenton reaction.