T-DM1 has an average DAR of 3.5 compared to an average of 8 for T-DXd.2 T-DXd has demonstrated efficacy in HER2 progressing on T-DM1 and has been shown to be effective and also approved for the treatment of breast cancer patients with low HER2 expression.18,20. This evidence concerns the gene ERBB2 and breast carcinoma.