NR4A1 and breast cancer: Endocrine therapy (ET) is a cornerstone treatment for patients with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2−) breast cancer, a subtype that accounts for two-thirds of all advanced or metastatic breast cancers (MBC).1 Treatment of MBC with ET is often augmented with targeted agents, including the small molecule inhibitors of cyclin-dependent kinase 4/6 (CDKi).2 The addition of CDKi (eg, palbociclib, ribociclib, and abemaciclib) to the treatment landscape have proven clinically beneficial.