TK1 and breast cancer: ,21 Studies have shown that patients with BC have higher levels compared with healthy controls22 and that higher serum TK1 activity is associated with poor BC outcomes.23, 24, 25 Intriguingly, TK1 kinetics during treatment have also been shown to be prognostic for outcomes in patients with metastatic BC treated with endocrine therapy,20,26 and for response to NACT.27