CDK4 and maternal uniparental disomy of chromosome 20: Up to now, the progress in scientific research has prompted us to better understand the pathophysiology of HR-positive and HER2-negative mBCs, so as to develop new drugs to strengthen ET, such as phosphoinositide 3 kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway inhibitors, histone deacetylase (HDAC) inhibitors and cyclin-dependent kinase 4/6 (CDK4/6) inhibitors (4).