Update and validate the performance of the 21-gene score in prognostic stratification for early-stage HR-positive, HER2-negative breast cancer in China.

Inclusion Criteria:

1. Diagnosed age: 18-75 years.
2. Early-stage invasive breast cancer, pN0-1.
3. Molecular subtype: HR-positive (ER and/or PR positive), HER2-negative.
4. At least five years of follow-up records (including cases with disease progression within five years and documented records).
5. Sufficient FFPE samples for sectioning or frozen samples for RNA extraction and subsequent testing.

Exclusion Criteria:

1. FFPE samples with tumor content <20%, and inability to enrich tumor regions through cutting.
2. Insufficient extracted nucleic acid quantity or severe degradation preventing further experiments.
3. Failure in RT-PCR amplification of housekeeping genes.
4. Multiple primary breast cancers or a history of ipsilateral or contralateral breast cancer.
5. Disease progression unrelated to breast cancer.
6. Lack of prior signed consent for retrospective scientific research or exemption from informed consent.

HR-positive, HER2-negative breast cancer