To gain a general understanding of the potential role of SUMO modification in ES cells, we reduced the SUMOylation level by knockdown of Sumo1/Ubc9, or increased the SUMOylation level by exogenously expressed Sumo1/Ubc9 in F9 embryonal carcinoma (F9 EC) cells. The gene discussed is UBE2I; the disease is embryonal carcinoma.