CDK4 and breast cancer: The following keywords were used: “breast cancer”, or “hormone receptor-positive breast cancer”, or “estrogen receptor-positive-breast cancer,” and “systemic therapy”, “targeted therapy”, “antiestrogen therapy”, “selective estrogen receptor modulator”, “selective estrogen receptor degraders”, “cyclin-dependent kinases 4 and 6 inhibitors”, and “aromatase inhibitors.” The studies that were published in English prior to December 2020 and limited to humans were assessed.