ERBB2 and breast cancer: CCK8 assay showed that trastuzumab-sensitive effect after GRB7 silencing could be partially restored by overexpression of circCDYL2 in both P-BT474 and P-SK-BR-3 cells (Fig. 4I), indicating that circCDYL2 promoted drug resistance of HER2+ BC cells via GRB7.