Overexpression of TCEAL9 was associated with lapatinib(IC50= 5.56 vs 10.90nM) and trastuzumab + pertuzumab(IC50= 745 vs 635nM) resistance in BT-474 and SK-BR-3 respectively...TCEAL9 expression correlates with trastuzumab resistance and high TCEAL9 expression is associated with poor prognosis in HER2-positive breast cancer patients.