The use of CDK4/6 inhibitors in combination with endocrine therapy improves survival in patients with hormone receptor-positive, HER2-negative metastatic breast cancer however further response biomarkers are required. In this interview, Andrew A. Davis, MD, Washington University School of Medicine, St. Louis, MO, discusses the development of a comprehensive next-generation sequencing (NGS)-based liquid biopsy assessment of ctDNA mutation and copy number analysis which can be used to identify novel prognostic and predictive biomarkers. This interview took place at the San Antonio Breast Cancer Symposium 2021 in San Antonio.