• Age 18 years or older.

• Patients with histologically confirmed breast cancer that is either

• A) HER2 positive as defined by 2018 ASCO-CAP guidelines determined by local testing B) Triple negative defined as ER negative (ER staining in \<1% tumour cells or ER Allred score \<3/8) and PR negative (PR staining in \<10% tumour cells or PR Allred score \<6/8) and HER2 negative by 2018 ASCO-CAP guidelines determined by local testing. Patients without PR testing results may enrol on the basis of ER and HER2 results.

• Note that patients negative for ER and PR may enrol whilst awaiting HER2 testing results

• Stage 1 cancer excluding pT1aN0 cancer, defined as

• A) Patients prior to surgery with primary tumour size on imaging 6-20mm and a normal axilla ultrasound or a biopsy negative axilla

• Patients who enrol prior to surgery will only continue further testing in the trial if their pathological staging fits the after surgery criteria.

• B) Patients after surgery with either

⁃ Primary tumour size pT1b or pT1c (6-20mm) and pN0 or pN1mi (micrometastasis).

⁃ Primary tumour size pT1a (1-5mm) and pN1mi

• Note that patients consenting after surgery may not enrol with pT1aN0 stage disease

• Patients should consent prior to surgery (preferred) or within 2 weeks of surgery. Patients who consent after surgery may extend consent to 4 weeks after surgery, although this will delay the receipt of ctDNA results.

• Planned and fit enough to receive full standard post-operative chemotherapy, with HER2 targeting as appropriate.

• Ability to give informed consent and comply with study procedures including blood tests and follow-up for five years.