In this report, we demonstrated that atorvastatin or the geranylgeranyltransferase (GGTase) I inhibitor GGTI-298 inhibited proliferation of the estrogen receptor (ER)-negative breast cancer MDA-MB-231 cells and down-regulated PBK, indicating that PBK is a target gene of geranylgeranylation signaling. The gene discussed is ESR1; the disease is breast cancer.