While ERα polyubiquitination promotes its degradation via the proteasome, ERα monoubiquitination leads to ERα stabilization by preventing its polyubiquitination; the RNF31 protein, which associates to ERα in the cytoplasm of BC cells and promotes its monoubiquitination, is responsible for increased ERα levels and enhanced ERα activity [69]. Here, ESR1 is linked to breast cancer.