ESR1 and breast carcinoma: With the use of sequence-specific antibodies against phosphorylation sites, analysis of protein phosphorylation profiles allows one to determine the activation status of signaling pathways, which can provide valuable prognostic insights.19–21 Atsriku et al undertook a systematic mapping of PTMs in the human estrogen receptor alpha (ER-α) in the MCF7 breast cancer cell line.