CDK4 and breast cancer: The USA Food and Drug Administration approved the use of sTK1 activity in 2022 as a biomarker for monitoring disease progression in previously diagnosed hormone receptor-positive (HR +), HER2-negative metastatic postmenopausal breast cancer(mBC) patients based on the results of the SWOG S0226 trial[7] and subsequently validated in a recent prospective trial of 287 mBC patients receiving first-line CDK4/6 inhibitor(CDK4/6i) in combination with endocrine therapy (ET) [8].