ESR1 and breast cancer: Posttranslational modifications of ERα, such as phosphorylation and poly-/mono-ubiquitination, appear to be central for the modulation of its stability, transport, and localization, and some may compete by the same site to modulate ERα stability and activity; for example, K303 is acetylated, mono- and poly-ubiquitinated in breast cancer cells (66, 73, 76, 113, 149), and some mutations at K303 exist in premalignant breast lesions (150, 151).