Embryonic stem cells (ESCs) were labeled with heavy isotopes, 13C615N4-arginine (Arg10) and 13C614N2-lysine (Lys6), or light isotopes, 12C614N4-arginine (Arg0) and 12C614N2-lysine (Lys0), respectively, for a total of 7 passages to ensure efficient labeling (>97%). The ESCs labeled with heavy isotopes were then treated with 20 mM SO for 24 hours before harvesting cells. Protein lysates of control and SO treated ESCs were prepared and mixed equivalently. Subsequently, trypsin digestion, high-performance liquid chromatography (HPLC) fractionation, Kla peptide enrichment with immobilized anti-Kla antibody, and high-resolution liquid chromatography–tandem MS (LC-MS/MS) were implemented. The resulting MS/MS data were processed using Maxquant search engine (v.1.5.2.8).