Guttery et al. 19 analysed ctDNA from 48 ER+ breast cancer patients receiving a variety of therapies and showed that they could detect ESR1 mutations in 11 patients, while Sefrioui et al. 20 analysed ctDNA and matched tissue from 7 ER+ patients and detected mutations in ctDNA in 4 of 6 patients with tumour mutations. This evidence concerns the gene ESR1 and breast cancer.