The forkhead transcription factor FoxA1 is thought to be involved in mammary tumorigenesis. However, the precise role of FoxA1 in breast cancer development is controversial. We examined expression of FoxA I in 35 human breast cancer cell lines and compared it with that of ErbB2, a marker of poor prognosis in breast cancer. We found that FoxA1 is expressed at high levels in all ErbB2-positive cell lines and a subset of ErbB2-negative cell lines. Down-regulation of FoxA1 by RNA interference significantly suppressed proliferation of ErbB2-negative and FoxA1-positive breast cancer cell lines. Down-regulation of FoxA1 also enhanced the toxic effect of Herceptin on ErbB2-positive cell lines through induction of apoptosis. Taken together with previous data that FoxA1 is a marker of luminal cells in mammary gland, our present results suggest that FoxA1 plays an important role as a lineage-specific oncogene in proliferation of cancer cells derived from mammary luminal cells. (c) 2007 Elsevier Inc. All rights reserved.