Trastuzumab deruxtecan efficacy in mouse tumor models resistant to T-DM1 was clearly observed, while, conversely, in NCI-N87, a HER2-positive gastric xenograft model with induced resistance to trastuzumab deruxtecan and tumor regression upon treatment with an MMAE-based trastuzumab ADC was observed. Here, ERBB2 is linked to neoplasm.