ESR1 and breast cancer: In light of the success of cyclin-dependent kinase 4 and 6 inhibitors in advanced HR-positive and HER2-triple-negative breast cancer, researchers have been striving to find more uses under different circumstances, including for early stage HR-positive and HER2-triple-negative breast cancer (Chou et al. 2020; Haddad et al. 2023) and for clinical breast oncology in other subtypes, such as HER2-positive breast cancer, HER2-positive estrogen receptor (ER)-positive breast cancer and triple-negative breast cancer (TNBC) (Blohmer et al. 2022; Tolaney et al. 2020).