In HER2-positive breast cancer, we focused on circEPSTI1, revealed the role of miR-145 as its downstream molecule, and verified that ERBB3 is regulated by miR-145 and has significant changes in mRNA and protein levels, which provides a basis for understanding the mechanism of the occurrence and development of HER2-positive breast cancer, and provides the therapeutic possibility of tumor drug resistance, recurrence, and metastasis. This evidence concerns the gene ERBB3 and breast cancer.