ESR1 and neoplasm: In addition, ctDNA analyses offer the possibility of non-invasive monitoring of tumour mutation status over time and treatment, an approach that has been used successfully in the identification of circulating KRAS mutations in colorectal patients receiving anti-epidermal growth factor receptor (EGFR)-targeted therapies21, but which has not been well explored in the context of ESR1 mutations and endocrine therapies.