Higher FLOT2 expression in nine HER2 amplified cell lines correlated with a higher T-DM1 IC50 <i>in vitro</i> , and breast cancer patients with high FLOT2 expression had worse survival when receiving either T-DXd (16.2 months (m) vs 18.3 m, p=0.04) or T-DM1 (38.0 m vs 41.3 m, p=0.1) in real-world Caris Life Sciences data. This evidence concerns the gene FLOT2 and breast cancer.