CDK4 and metastatic neoplasm: Addition of CDK4/6i to endocrine therapy (ET) has demonstrated superiority in several randomized controlled trials (RCTs) in phase III and IV with an increased progression-free survival (PFS) compared to ET monotherapy across various patient subgroups including patients with de novo and recurrent metastatic disease, pre- and postmenopausal women, progesterone receptor negative disease, bone- and visceral-metastases.