Potential through simplicity: thymidine kinase-1 as a biomarker for CDK4/6 inhibitors.

We describe a potential role for thymidine kinase-1, a general marker of cellular proliferation, to act as a prognostic biomarker in patients receiving CDK4/6 inhibitors for advanced hormone receptor-positive, HER2-negative breast cancer, with early data suggesting that it may also provide early indication of treatment response.