Atorvastatin inhibited cell proliferation through inhibition of geranylgeranyl biosynthesis in MDA-MB-231, an ER-negtive (ER-) breast cancer cell line. The ER- breast cancer MDA-MB-231 cells or the ER+ breast cancer MCF7 cells were treated with atorvastatin (10 μM) or atorvastatin (10 μM) plus GGOH (10 μM) for 48 hrs. The cells were photographed under a phase microscope and counted with a hemocytometer. The proliferation bar graphs were counted from three independent experiments. A and B, MDA-MB-231 cells; C and D, MCF7 cells.