Recent studies have suggested that autocrine production of Neuregulin (NRG), a growth factor that activates members of the Epidermal Growth Factor Receptor/ErbB family of proto-oncogenes, is sufficient for breast tumor initiation and progression. To elucidate the molecular mechanisms regulating these events, we undertook a global analysis of genes regulated by NRG in luminal mammary epithelial cell lines. Gene expression profiling of estrogen receptor-positive T47D cells exposed to NRG-1 revealed both previously identified and novel targets of NRG activation. Profiling of other estrogen receptor-positive breast cancer cell lines, MCF7 and SUM44, yielded a group of twenty-one genes whose transcripts are upregulated by NRG in all three lines tested. The NRG targets are FBJ murine osteosarcoma viral oncogene homolog B, Early growth response 1, v-jun avian sarcoma virus 17 oncogene homolog, Activating transcription factor 3, Homo sapiens cDNA FLJ31636 fis, Jun B proto-oncogene, Forkhead box C1, Platelet/endothelial cell adhesion molecule 1, NADPH-dependent retinol dehydrogenase/reductase, Dual specificity phosphatase 5, NGF inducible protein TIS21, Connective tissue growth factor, Jun D proto-oncogene, Serum response factor, Cullin 1, v-myc avian myelocytomatosis viral oncogene, Transient receptor potential channel 1, Low density lipoprotein receptor, Transforming growth factor beta 1, Nucleoporin 88 kDa, and Pleckstrin homology-like domain A1. Since NRG activation of these cells induces resistance to anti-hormonal therapy, the identified genes may provide clues to molecular events regulating mammary tumor progression and hormone independence.