Circulating tumor DNA (ctDNA) from a liquid biopsy (LB) is particularly well suited for this purpose, and several studies have shown that ESR1 mutations could be detected from a blood-based LB months before clinical disease progression, and the polyclonal nature of breast cancer metastases can be better ascertained with a LB [13,14,18,19]. Here, ESR1 is linked to breast carcinoma.