This was the first-in-human clinical study to evaluate the safety, pharmacokinetics (PK), pharmacodynamics, and preliminary antitumor activity of PF-06873600, alone and in combination with ET, in patients with HR+/HER2− advanced breast cancer or metastatic breast cancer (mBC) who were CDK4/6i-naïve but had prior ET or had failed prior combination CDK4/6i and ET. The gene discussed is CDK4; the disease is breast carcinoma.