Recent results from the SONIA trial [28, 29] which tested the benefit of 1st-line CDK4/6i versus 2nd-line CDK4/6i by randomizing women with HR+/HER2 advanced breast cancer to either (1) 1st-line aromatase inhibitor + CDK4/6i followed by 2nd-line fulvestrant alone or (2) 1st-line aromatase inhibitor alone followed by 2nd-line CDK4/6i + fulvestrant. This evidence concerns the gene CDK4 and breast cancer.