To investigate changes in gene expression associated with ERBB2, expression profiling of immortalized human mammary luminal epithelial cells and variants expressing a moderate and high level of ERBB2 has been carried out using cDNA microarrays corresponding to approximately 6000 unique genes/ESTs. A total of 61 significantly up- or downregulated (2.0-fold) genes were identified and further validated by RT-PCR analysis as well as microarray comparisons with a spontaneously ERBB2- overexpressing breast cancer cell line and ERBB2-positive primary breast tumors. The expression and clinical relevance of proteins predicted to be associated with ERBB2 overexpression in breast cancers were analysed together with their clinical relevance by antibody screening using a tissue array. Differentially regulated genes include those involved in cell-matrix interactions including proline 4-hydroxylase (P4HA2), galectin 1 (LGALS1) and galectin 3 (LGALS3), fibronectin 1 (FN1) and p-cadherin (CDH3), and cell proliferation (CRIP1, IGFBP3) and transformation (S100P, S100A4). A number of genes associated with MYC signalling were also differentially expressed, including NDRG1, USF2 and the epithelial membrane proteins 1 and 3 (EMP1, EMP3). These data represent profiles of the transcriptional changes associated with ERBB2-related pathways in the breast, and identify novel and potentially useful targets for prognosis and therapy.