circCDYL2 promotes trastuzumab resistance by maintaining HER2 downstream signal transduction.Specifically, circCDYL2 stabilizes GRB7 forming a circCDYL2-GRB7-FAK complex, and maintaining the activity of downstream signaling molecules AKT and ERK1/2.Studies have shown that HER2+ breast cancer patients with high expression of circCDYL2 have poorer outcomes and shorter disease-free survival (DFS) and overall survival (OS) after trastuzumab treatment. Here, PTK2 is linked to breast cancer.