CDK4 and breast cancer: For example, the receptor tyrosine kinase HER2 is targeted by the monoclonal antibody therapy trastuzumab in HER2 breast cancers [5], CDK4/6 by palbociclib in endocrine therapy-resistant luminal breast cancers [6] and particular FGFR family members represent potential therapeutic targets in subsets of luminal and triple negative breast cancer patients [7].