ESR1 and breast cancer: ERα phosphorylation, first described over three decades ago, is unquestionably the best studied of all its PTMs, particularly those most commonly observed serine (S) phosphorylation events in ER-positive breast cancer cells detectable by either site-specific antibodies or modern mass spectrometry approaches [4–7]: phosphorylation of the receptor's N-terminal (AF-1) domain at S118 and S167 [8, 9], and its more recently described hinge and ligand-binding domain (LBD) phosphorylation at S294 and S305 [10, 11].