Breast cancer is one of the most common human neoplasms, accounting for 22% of all cancers in women worldwide. (1) The incidence rate is higher in North America, Europe, and Australia than in the less developed regions of Africa and Southern and Eastern Asia. Breast cancer is a highly heterogeneous group of cancers, with diversity in its morphology, molecular genetics, biology, and clinical outcome. Conventionally, invasive human breast carcinomas have been classified morphologically into infiltrating ductal and lobular carcinoma, tubular carcinoma, mucinous carcinoma, medullary carcinoma, invasive papillary carcinoma, metaplastic carcinoma, and a few uncommon types. Recently, gene expression profiling, a method using cDNA microarray to explore gene expression patterns, has classified breast cancer into 5 distinct subtypes based on variations in gene expression patterns. These 5 subtypes are luminal A and luminal B, normal breastlike, human epithelial growth factor receptor 2 (HER2) overexpressing, and basal-like subtypes. (2) Each subtype of tumors expresses a distinct set of genes. Basal-like subtype was characterized by high expression of cytokeratins (CKs) 5 and 17, laminin, and fatty acid-binding protein 7, whereas HER2 overexpressing was characterized by high expression of several genes in the ERBB2 amplicon at 17q22.24, including ERBB2 and GRB7 (Table). In this classification, tumors with the same gene expression pattern are grouped together, which provides a mechanism for studying the association of gene expression profiles with tumor morphology, biologic behavior, response to therapy, and clinical outcome of patients.