Despite the success of protein farnesyltransferase inhibitors (FTIs) in the treatment of certain malignancies, their mode of action is incompletely understood. Dissecting the molecular pathways affected by FTIs is important, particularly because this group of drugs is now being tested for the treatment of Hutchinson-Gilford progeria syndrome. In the current study, we show that FTI treatment causes a centrosome separation defect, leading to the formation of donut-shaped nuclei in nontransformed cell lines, tumor cell lines, and tissues of FTI-treated mice. Donut-shaped nuclei arise during chromatin decondensation in late mitosis; subsequently, cells with donut-shaped nuclei exhibit defects in karyokinesis, develop aneuploidy, and are often binucleated. Binucleated cells proliferate slowly. We identified lamin B1 and proteasome-mediated degradation of pericentrin as critical components in FTI-induced "donut formation" and binucleation. Reducing pericentrin expression or ectopic expression of nonfarnesylated lamin B1 was sufficient to elicit donut formation and binucleated cells, whereas blocking proteasomal degradation eliminated FTI-induced donut formation. Our studies have uncovered an important role of FTIs on centrosome separation and define pericentrin as a (indirect) target of FTIs affecting centrosome position and bipolar spindle formation, likely explaining some of the anticancer effects of these drugs.cell division | nuclear envelope | doughnut-shaped nuclei | antitumor P rotein farnesylation is a posttranslational modification that facilitates the binding of proteins to membrane surfaces. Protein farnesyltransferase catalyzes the addition of a 15-carbon farnesyl lipid to proteins containing a carboxyl-terminal CaaX motif consisting of a cysteine (C) followed by two aliphatic amino acids (aa) and a terminal amino acid residue (X), which is often alanine, serine, methionine, or glutamine (1). Among the most familiar examples of farnesylated proteins are the Ras family of proteins, which require the farnesyl lipid for anchoring the protein to the plasma membrane and for proper protein function. Because mutations in RAS oncogenes are involved in ∼30% of all human cancers and are associated with poor prognosis and treatment outcome (2, 3), protein farnesyltransferase was seen as an attractive target for anticancer drug therapy, prompting the development of many protein farnesyltransferase inhibitors (FTIs). Interestingly, FTIs exhibit significant efficacy in tumor cells in animal models whether or not they have RAS mutations, suggesting that the efficacy of FTIs cannot be attributed solely to their effects on Ras processing and raising the possibility that other mechanisms underlie their anticancer properties. Recently, FTIs have found another potential application in HutchinsonGilford progeria syndrome (HGPS). HGPS is a pediatric progeroid disorder caused by a mutant form of prelamin A that (unlike mature lamin A) retains its farnesyl lipid anchor. Based on salutary effects of FTIs on disease phenotypes in...