ESR1 and breast cancer: Because TK1 is a cell cycle-regulated enzyme that plays a critical role in DNA synthesis, we investigated whether serum TK1 activity could be used as a surrogate marker of the antiproliferative effect of palbociclib in patients with early-stage ER+/HER2− breast cancer enrolled in the NeoPalAna trial (neoadjuvant palbociclib and anastrozole).