FMK-9a, with an IC50 of 80 nM, is the most potent ATG4B inhibitor so far reported, but it could neither reduce the survival rate of cancer cells nor improve the anti-tumor effect of chemotherapeutic drugs within a reasonable concentration range (Fu and Li, unpublished results), suggesting that the antitumor effect of ATG4B inhibitors may not necessarily be correlated with the inhibition of ATG4B enzymatic activity as measured by in vitro assays. Here, ATG4B is linked to cancer.