The development of inhibitors of CDK4 and CDK6 has changed the perception of CDKs as therapeutic targets in breast cancer following “underwhelming results and unacceptable toxicity were seen with pan-CDK inhibitors such as flavopiridol (alvocidib) in the early 2000s” (Fry et al., 2004; Toogood et al., 2005; Hosford and Miller, 2014; Asghar et al., 2015). This evidence concerns the gene CDK4 and breast cancer.