ESR1 and breast carcinoma: In breast cancer patients, ctDNA levels can predict for pCR (Rothé et al., 2019), relapse (>6 months before clinical detection) (Garcia-Murillas et al., 2015) and MRD (McDonald et al., 2019) and have been investigated as a predictive biomarker to guide targeted therapies (Turner et al., 2020b), immunotherapies and monitoring resistance, e.g., to known mechanisms such as ESR1 mutations (Schiavon et al., 2015).