DiviTumTKa is an in vitro diagnostic device intended for the semi-quantitative measurement of thymidine kinase activity (TKa) in human serum. The assay is to be used as an aid in monitoring disease progression in previously diagnosed hormone receptor positive, metastatic postmenopausal female breast cancer patients. A TKa value of < 250 DuA is associated with the decreased likelihood of disease progression within 30 days or 60 days post testing. DiviTumTKa results should be used in conjunction with other clinical methods for monitoring breast cancer.