ERBB2 and breast cancer: Indeed, in vitro studies conducted in estrogen receptor (ER)-positive cell lines, such as MCF-7 and T47-D, and also in HER2-positive BC cell lines, demonstrated that CDK4/6 inhibitors were capable of limiting proliferation and inducing cell cycle arrest in both cell lineages, providing a strong and robust rationale for CDK4/6 inhibitors’ development in clinical practice [8].