In a breast cancer patient-derived xenograft (PDX) model, DS-8201a (DAR 8) at a dose of 10 mg/kg exhibited impressive antitumor activity against a low HER2-expressing tumor, while T-DM1 (DAR 3.5) displayed virtually no cytotoxicity against the same tumor at similar doses [63]. The gene discussed is ERBB2; the disease is breast cancer.