This study is a multicenter, observational, real-world investigation. The research plans to enroll 300 HER2-positive and HER2-low breast cancer patients who meet the inclusion criteria. All patients included in the analysis are currently receiving or are scheduled to receive a treatment regimen containing Trastuzumab-rezetecan. There are no restrictions on the treatment regimen, which is entirely based on the clinician's choice. The study aims to evaluate the efficacy and safety of the treatment regimen containing Trastuzumab-rezetecan.

After completing screening examinations and assessments, eligible patients will enter the study treatment phase. They will receive the treatment regimen containing Trastuzumab-rezetecan and undergo follow-up visits according to the protocol. During the study treatment period, patients will undergo imaging and safety assessments as per clinical routine, with the investigator's assessment serving as the final result. Upon treatment completion or study withdrawal, corresponding safety examinations and imaging assessment data will be collected. For neoadjuvant patients, pathological complete response (pCR) will be assessed post-surgery by pathologists at the participating centers.

Inclusion Criteria:

1. Age ≥ 18 years, with pathologically confirmed HER2-positive or HER2-low-expressing breast cancer: HER2-positive is defined as IHC 3+ or IHC 2+ with a positive ISH result; HER2-low-expressing is defined as IHC 1+ or IHC 2+ with a negative ISH result for HER2 amplification; hormone receptor status must be clearly documented.
2. Planned to receive treatment with Trastuzumab-rezetecan for Injection in the neoadjuvant, adjuvant, or advanced/metastatic setting; patients with advanced/metastatic disease must have progressed after at least one prior line of systemic therapy.
3. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0-2.
4. Women of childbearing potential must have a negative serum pregnancy test within 28 days prior to enrollment, and both male and female patients must be willing to use a highly effective, medically approved method of contraception during the study and for 1 year after the last dose of the study drug.
5. Non-pregnant and non-lactating female patients.
6. Not concurrently participating in any other ongoing clinical studies. The patient or their legal representative has provided informed consent by signing the informed consent form and is willing and able to comply with scheduled visits, the study treatment plan, laboratory tests, and other trial procedures.

Exclusion Criteria:

1. Any evidence of severe or uncontrolled systemic disease, including uncontrolled hypertension, active bleeding disorders, active infections (such as hepatitis B, hepatitis C, and human immunodeficiency virus), or severely impaired bone marrow reserve or organ function (including hepatic and renal impairment), which, in the investigator's opinion, would significantly alter the risk-benefit balance.
2. At the initiation of treatment with Trastuzumab-rezetecan for Injection, the patient has not recovered from any prior treatment-related toxicity of CTCAE grade >3.
3. Known history of hypersensitivity to Trastuzumab-rezetecan for Injection, its excipients, or drugs with similar chemical structures or classes as Trastuzumab-rezetecan.
4. Patients deemed unsuitable for enrollment by the investigator.

HER2-positive and HER2-low breast cancer patients, including all those analyzed who are currently receiving or plan to receive a treatment regimen containing Trastuzumab-rezetecan.