Metabolic engineering of heterologous pathways has allowed the production of therapeutically important compounds in microbial systems. Here, we report the engineering of a monoterpenoid biosynthetic pathway into Escherichia coli. Five genes encoding sequential enzymes for perillyl alcohol biosynthesis from the precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) were engineered into E. coli. Expression of these genes allowed the production of the intermediate limonene, but the downstream monoterpenoid, perillyl alcohol, was not detected. A new compound was detected but could not be identified based on the data obtained. Only 1.6 mu g/ml of the compound was being produced from the engineered E. coli strain, but, when these cultures were fed limonene as a substrate, the production was nearly 250 mu g/ml. This unknown compound inhibited the cell proliferation of MCF-7 and MDA-MB-231 breast cancer cells in 48-h treatment experiments. This compound may have potential benefits in breast cancer treatment. This is the first report showing the production of a monoterpenoid in engineered E. coli and its antiproliferative effects in breast cancer cells.