These gene expression analysis of single cell preparations confirmed that ER drives cell cycle progression in ERhigh cells; CDK4/6 inhibitors, such as palbociclib, are effective mainly in ERhigh cells which target the cell cycle; and these inhibitors target the ER+/HER2+ cancer when the tumors express high levels of ER and use the ER pathway for cell growth. Here, CDK4 is linked to cancer.