Lim-Only Protein 4 (LMO4) regulates luminal differentiation and proliferation in the developing mammary gland. More than 50% of human breast tumours abnormally overexpress LMO4 with high expression correlated with a poor patient prognosis. Overexpression of LMO4 arises through increased promoter activity and the aberrant activation of the second promoter. Pinpointing the direct and indirect pathways that regulate the expression of LMO4 will help address how LMO4 functions in oncogenesis and development. To this end, a genome-wide siRNA screen was conducted in which breast cancer cells were modified to carry a luciferase reporter gene that is driven by the LMO4 promoter. The primary phase of screening identified 27 positive regulators with transcription factor activity and 307 negative regulators. 130 candidates validated through the secondary phase and subsequent to this we identified candidates whose knockdown reproducibly modulated LMO4 expression in both T47D and MCF7 cells. We have identified 24 high confidence candidates for continued study. Candidates that show the ability to reduce LMO4 at the protein level will have the potential to be further developed as therapeutic targets for breast tumours that overexpress LMO4. In addition, experiments are in progress to determine if LMO4 has a role in tumour metastasis, and to identify potential roles in metastasis for candidates identified in the screen.