Background Id3 plays a key role in the progression of breast cancer.Previously,four and a half LIM protein (FHL2) was identified as a repressor of Id family proteins by interacting with them.This study aimed to investigate the effects of FHL2 on the transcriptional regulation and oncogenic activities of Id3 in human breast cancer cells.Methods Cell transfection was performed with SuperFect reagent.Stable transfectants that overexpressed Id3 were obtained by selection on G418.The level of Id3 protein was determined by Western blotting analysis.Dual luciferase assays were used to measure the effect of Id3 and FHL2 on E47-mediated transcriptional activity in MCF-7 human breast cancer cells.The MTT assay was used to measure cell proliferation.The transwell assay was used to measure the invasive capacity of MCF-7 cancer cells.Results Id3 markedly repressed transcription mediated by the basic helix-loop-helix (bHLH) factor E47 in MCF-7 cells.This Id3-mediated repression was effectively antagonized by FHL2.Overexpression of Id3 markedly promoted the proliferation and invasive capacity of MCF-7 cells; however,these effects were significantly suppressed by the overexpression of FHL2.Conclusions FHL2 can inhibit the proliferation and invasive growth of human breast cancer cells by repressing the functional activity of Id3.The functional roles of FHL2-1d3 signaling in the development of human breast cancer need further research.