Except the currently used estrogen receptor status of breast cancers, no reliable biomarkers could be defined to diagnose tumors that depend on CDK4 activity and hence would respond to CDK4/6 inhibitors (Dickson, 2014; Asghar et al, 2015; DeMichele et al, 2015; Sherr et al, 2016). This evidence concerns the gene ESR1 and breast cancer.