To determine whether serum TK1 activity could serve as a surrogate marker for CDK4/6 inhibition and tumor cell proliferation in patients receiving CDK4/6 inhibitors, we analyzed the sample set collected from patients with clinical stage II-III ER+/HER2− breast cancer who received neoadjuvant anastrozole and palbociclib in the NeoPalAna trial [19]. Here, ESR1 is linked to breast cancer.