RATIONALE: Studying samples of tissue from patients with cancer in the laboratory may help doctors learn more about changes that occur in DNA and identify biomarkers related to cancer.

PURPOSE: This research trial studies biomarkers as a predictor of response to trastuzumab in samples from patients with breast cancer previously treated in the NSABP-B-31 trial.

OBJECTIVES:

Primary

• To determine if stem cellness identifies a poor prognostic subgroup of women with early-stage breast cancer who have been uniformly treated with either adjuvant doxorubicin hydrochloride & cyclophosphamide followed by paclitaxel (the "control arm" of B31), or the same chemotherapy plus trastuzumab.

Secondary

• To conduct exploratory analyses to assess, to the extent possible, if co-localization of stem cellness, as determined by ALDH1 positivity, and HER2 identifies a group of patients previously considered to have HER2-negative cancers (using classical definitions) who benefit from adjuvant trastuzumab.

OUTLINE: Archived breast cancer stem cells samples and terminally differentiated cells from tissue samples are analyzed for HER2 and ALDH1 expression by dual-staining quantitative immunofluorescence using Automated Quantitative Analysis (AQUA) , IHC, and fluorescence in situ hybridization (FISH).

DISEASE CHARACTERISTICS:

• Tissue samples from patients with node-positive breast cancer whose tumors overexpress HER2
• Primary tumor samples that are negative for HER2 using classically accepted cutoffs determined in the metastatic setting
• Treated with adjuvant therapy comprising doxorubicin hydrochloride, cyclophosphamide, and paclitaxel with or without trastuzumab in the NSABP-B-31 trial

PATIENT CHARACTERISTICS:

• Not specified

PRIOR CONCURRENT THERAPY:

• See Disease Characteristics

Primary cancer tissue from participants in protocol NSABP B-31.