原文文献：

Caliskan Yildirim E, Atag E, Coban E, Umit Unal O, Celebi A, Keser M, Uzun M, Keskinkilic M, Tanrikulu Simsek E, Sari M, Yavuzsen T. The effect of low HER2 expression on treatment outcomes in metastatic hormone receptor positive breast cancer patients treated with a combination of a CDK4/6 inhibitor and endocrine therapy: A multicentric retrospective study. Breast. 2023 Aug;70:56-62. doi: 10.1016/j.breast.2023.06.006. Epub 2023 Jun 14. PMID: 37343321; PMCID: PMC10382953.