Interestingly, early results from a neoadjuvant trial of palbociclib for patients with early stage luminal BC have shown that serum TK1 levels were highly concordant with Ki-67 assessed by immunohistochemistry on tumor tissue biopsies, suggesting that TK1 may represent a valid pharmaco-dynamic marker in this context [28] that can be serially tested in the blood without the need of tumor tissue. This evidence concerns the gene TK1 and breast cancer.