The objectives of this study were to compare TK1 activity in serum collected before and after anastrozole and palbociclib, and to correlate serum TK1 activity with tumor Ki-67 proliferation index and tumor TK1 mRNA levels in patients with early-stage ER+/HER2− breast cancer enrolled in the NeoPalAna trial. Here, ESR1 is linked to neoplasm.