The prognostic role of serum TK1 assessed at baseline and on treatment is being further explored in two ongoing clinical trials of luminal BC patients treated with CDK4/6 inhibitors and ET: BIOITALEE (NCT03439046), a Phase 3b biomarker study of ribociclib plus letrozole in the first-line setting and PYTHIA (NCT02536742), a phase 2 biomarker discovery trial of palbociclib and fulvestrant in patients with endocrine resistant disease. The gene discussed is CDK4; the disease is breast cancer.