Purpose of Review <p>Thymidine kinase has emerged as a promising blood-based biomarker in breast cancer, which can be measured using the ELISA-based assay, DiviTum^®. This review explores thymidine kinase’s potential as a pharmacodynamic biomarker in both early and late-stage breast cancer, its role in disease monitoring, and its potential future use in guiding treatment decisions.</p> Recent Findings <p>In late-stage Estrogen Receptor Positive, HER2 negative breast cancer, baseline and early use of thymidine kinase activity levels during treatment appear to be both prognostic and predictive of response to CDK4/6 inhibitor therapy. Additionally, longitudinal monitoring of thymidine kinase activity may detect progression prior to radiographic progression.</p> Summary <p>Thymidine kinase may be a valuable tool for prognosticating outcome as well as predicting response to CDK4/6 inhibitors and longitudinal disease monitoring in late-stage Estrogen Receptor Positive, HER2 negative breast cancer. Looking ahead, thymidine kinase could have the potential to become an important biomarker in Estrogen Receptor Positive, HER2 negative breast cancer to help personalize treatment decisions.</p>