BL-B01D1 is an ADC that binds a surface antigen (e.g., TROP2/EGFR/HER3), is internalized, and releases a topoisomerase I–inhibitor payload that causes DNA damage and apoptosis. DNA topoisomerase I is the intracellular enzymatic target of the payload, not the binding antigen; cells aren’t selectively killed based on topoisomerase I expression alone.