IDEA (Individualized Decisions for Endocrine Therapy Alone) is an American, multicentre, single-arm prospective cohort study headed by the University of Michigan Rogel Cancer Centre, enrolling postmenopausal women (50–69 years) with ER+/PR+/HER2−, unifocal, pT1N0M0 breast cancer. This evidence concerns the gene PGR and breast cancer.