Circular RNAs (circRNAs) are noncoding RNAs that form covalently closed loop 
structures. CircRNAs are dysregulated in cancer and play key roles in 
tumorigenesis, diagnosis, and tumor therapy. CircRNAs function as competing 
endogenous RNAs or microRNA sponges that regulate transcription and splicing, 
binding to proteins, and translation. CircRNAs may serve as novel biomarkers for 
cancer diagnosis, and they show potential as therapeutic targets in cancers 
including breast cancer (BC). In women, BC is the most common malignant tumor 
worldwide and the second leading cause of cancer death. Although evidence 
indicates that circRNAs play a critical role in BC, the mechanisms regulating 
the function of circRNAs in BC remain poorly understood. Here, we provide 
literature review aiming to clarify the role of circRNAs in BC and summarize the 
latest research. We provide a systematic overview of the biogenesis and 
biological functions of circRNAs, elaborate on the functional roles of circRNAs 
in BC, and highlight the value of circRNAs as diagnostic and therapeutic targets 
in BC.