The authors used a transgenic MMTV/Neu (Erbb2) mouse model of breast cancer to confirm that blockade of SUMOylation, using anacardic acid (E1‐SUMO intermediate inhibitor) or conditional knockout of Ube2i (encoding E2 enzyme Ubc9), extends tumor‐free survival with no apparent deficiencies in normal mammary gland development and stem cell activity. Here, UBE2I is linked to neoplasm.