We therefore explored the value of baseline and early 18F-FDG PET/CT evaluations in predicting progression free survival (PFS) in a larger consecutive series of ER+ / HER2- mBC patients treated with CdK4/6i in a specialist breast center certified by the European Society of Breast Cancer Specialists (EUSOMA) over the period of inclusion [17]. This evidence concerns the gene ESR1 and breast cancer.