Abstract: Cell cycle inhibitors improve the quality of life and prognosis in estrogen receptor (ER)-positive breast cancer. Breast cancer featuring low expression of ER differs from breast cancer with high expression of ER. The efficacy of endocrine therapy is poor for the low ER cases. With the development of detection technology and drug resistance mechanisms, the epidemiology, clinicopathological characteristics, and treatment strategies for patients with low ER expression have attracted the attention of scholars. In this article, the progress made in research on breast cancer with low ER expression was reviewed.