In non-tumorigenic mammary epithelial cells, HDAC1 is preferentially conjugated to Sumo1 leading to HDAC1 proteolysis, whereas, in breast cancer cells, HDAC1 is predominantly conjugated to Sumo2, promoting HDAC1 protein stability; the specificity for SUMO proteins is dictated by the PIASγ ligase. The gene discussed is HDAC1; the disease is breast cancer.