Observational study in HER2‑low breast cancer patients receiving anti‑HER2 antibody–drug conjugates (ADCs, e.g., trastuzumab deruxtecan). Drug type/mechanism: ADCs are monoclonal antibodies linked to a cytotoxic payload that bind HER2 (ERBB2) on tumor cells, undergo endocytosis, and are processed in lysosomes to release the payload, inducing tumor cell apoptosis while limiting normal‑tissue damage. Targets: HER2‑expressing breast cancer cells, HER2/ERBB2 signaling, endosomal–lysosomal trafficking for payload release. Intervention/assessment: serial serum HER2 (shed extracellular domain) measurements before therapy and after 2, 4, and 6 cycles to correlate changes with ADC treatment efficacy and guide monitoring of response or resistance.