• Ability to give written informed consent

• Age \> 18 years at time of study entry.

• Patient would be willing to undergo surgery is appropriate for surgery

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 (Appendix 1).

• Tumor size \> 1 cm to ≤ 3 cm assessed by ultrasound and clinically and radiographically node negative with no known metastatic disease.

• HER2+ BC as defined by American Society of Clinical Oncology-College of American Pathologists (ASCO-CAP) guidelines.31 Patients may have ER+ or ER- negative disease, as defined by ASCO-CAP guidelines.

• Left ventricular ejection fraction (LVEF) must be within institutional limits of normal as assessed by echocardiogram (ECHO) or multigated acquisition (MUGA) scan, documented within 4 weeks prior to first dose of study drug.

• Adequate normal organ and marrow function as defined below:

‣ Hemoglobin ≥ 9.0 g/dL

⁃ Absolute neutrophil count (ANC) ≥ 1.5 x 109/L (≥ 1500 per mm3)

⁃ Platelet count ≥ 100 x 109/L (≥100,000 per mm3)

⁃ Serum bilirubin ≤ 1.5 x institutional upper limit of normal (ULN). The maximum allowable bilirubin is ≤ 2.5 x ULN for patients with Gilbert's disease.

⁃ Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3 x institutional ULN

⁃ Calculated glomerular filtration rate \>50 mL/min

• Patients must either be of non-reproductive potential or willing to undergo appropriate contraception. Male subjects must agree not to donate sperm and female subjects must agree not to donate oocytes starting at screening and throughout the study period, and for at least 12 months after treatment discontinuation.

⁃ Patient with reproductive potential must have a negative pregnancy test ≤3 days prior to the first dose of zanidatamab.