CDK4 and breast cancer: While palbociclib and other CDK4/6 inhibitors have been approved by FDA in breast cancer in combination with endocrine therapy [3], our goal was to determine if [18F]FLT should be considered a potential in vivo biomarker for predictive response in other tumors since CDK4/6 inhibitors are increasingly being considered in other solid cancers either as single agents or in combination therapy [17, 18].