Polyethylene glycol maleimide (PEG‐mal) is commonly used to modify proteins by alkylating free cysteine residues that are not engaged in disulfide bonds.[25] Adding PEG‐mal to the protein lysates of KYSE140 and KYSE510 cells revealed that the molecular weight of TSC2 in control cells was higher than that in QSOX2‐overexpressed ESCC cells, indicating that there were fewer free cysteine residues and more disulfide bonds formed after QSOX2 overexpression (Figure 5G). Here, TSC2 is linked to esophageal squamous cell carcinoma.