Abstract: About 70% to 80% of breast cancers express estrogen receptor-? (ER?), and estrogens play important roles in the development and growth of hormone-dependent tumors. Together with lymph node metastasis, tumor size and histological grade, ER status is considered one of the prognostic factors in breast cancer, and an indicator for hormonal treatment. 147 genes and 112 genes with significant P-value and having significant differential expression between ER+ and ER? tumors were identified from the LCM data set and bulk tissue data set, respectively. 61 genes were found to be common in both data sets, while 85 genes were unique to the LCM data set and 51 genes were present only in the bulk tumor data set. Pathway analysis with the 85 genes using Gene Ontology suggested that genes involved in endocytosis, ceramide generation, Ras/ERK/Ark cascade, and JAT-STAT pathway may play roles related to ER.