Inboth BT-474 and BT-474-TR cells, anti-HER2 VHH-FcK showed considerablylower viability compared with Trastuzumab across a range of concentrations.The results obtained from the MTT assay, conducted with antibody treatmentin the absence of immune cell, indicate that anti-HER2 VHH-FcK hasinherent cytotoxicity against HER2-positive breast cancer includingTrastuzumab-resistant HER2-positive breast cancer. Here, ERBB2 is linked to breast carcinoma.