CDK4 and breast cancer: Treatment for patients with breast cancer has evolved in the last years and recently approved targeted treatments received by the patients included cyclin-dependent kinase 4/6 (CDK4/6) inhibitors, immune checkpoint inhibitors, poly ADP ribose polymerase (PARP) inhibitors, phosphoinositide 3-kinase (PI3K) inhibitors, antibody-drug conjugates and investigational drugs.