CYP19A1 and breast cancer: PALLET is a randomised phase II trial in early breast cancer aiming to investigate whether the addition of palbociclib (a targeted drug that blocks CDK4/6 proteins involved in cell division) to letrozole (an aromatase inhibitor) in the neoadjuvant setting (prior to surgery) can reduce tumour size and assess the effect of treatment on the reduction of tumour cell proliferation by measuring Ki67 at multiple time points.