ERBB2 and breast cancer: In a BT-474 HER2-positive breast cancer xenograft model, a single dose of 5 mg/kg HT-19 antibody was ineffective, while a single dose of 2 mg/kg or 5 mg/kg XMT-1522 induced durable complete tumor regression, indicating that the primary mechanism of XMT-1522 is cytotoxic payload delivery, not HER2 signaling inhibition [45].