The human cytomegalovirus (CMV) US2-US11 genomic region contains a cluster of genes whose products interfere with antigen presentation by the major histocompatibility complex (MHC) proteins. Although included in this cluster, the US9 gene encodes a glycoprotein that does not affect MHC activity and whose function is still largely uncharacterized. An in silico analysis of the US9 amino-acid sequence uncovered the presence of an N-terminal signal sequence (SS) and a C-terminal transmembrane domain containing the specific hallmarks of known mitochondrial localization sequences (MLS). Expression of full-length US9 and of US9 deletion mutants fused to GFP revealed that the N-terminal SS mediates US9 targeting to the endoplasmic reticulum (ER) and that the C-terminal MLS is both necessary and sufficient to direct US9 to mitochondria in the absence of a functional SS. This dual localization suggested a possible role for US9 in protection from apoptosis triggered by ER-to-mitochondria signalling. Fibroblasts infected with the US2-US11 deletion mutant virus RV798 or with the parental strain AD169varATCC were equally susceptible to death triggered by exposure to tumour necrosis factor (TNF)-a, tunicamycin, thapsigargin, brefeldin A, lonidamine and carbonyl cyanide m-chloro phenyl hydrazone, but were 1.6-fold more sensitive to apoptosis induced by hygromycin B. Expression of US9 in human embryonic kidney 293T cells or in fibroblasts, however, did not protect cells from hygromycin B-mediated death. Together, these results classify US9 as the first CMV-encoded protein to contain an N-terminal SS and a C-terminal MLS, and suggest a completely novel role for this protein during infection. INTRODUCTIONHuman cytomegalovirus (CMV) is a very prevalent betaherpesvirus that poses serious medical concerns in immunocompromised individuals . The ability to evade or modulate host immune responses, coupled with the capacity to establish latency after primary infection, is at the basis of the tremendous success of CMV as a human pathogen.The US2-US11 genomic region has been shown to be dispensable for viral replication in primary human fibroblasts (HF) (Jones & Muzithras, 1992;Kollert-Jons et al., 1991), and to encode five endoplasmic reticulum (ER)-resident glycoproteins, US2, US3, US6, US10 and US11, that disrupt antigen presentation by the major histocompatibility complex (MHC) class I and class II proteins (Lin et al., 2007). Despite sharing some very limited homology with US6, US10 and US11, US9 does not bind to or reduce MHC cell surface levels (Hegde et al., 2002(Hegde et al., , 2006Pereira et al., 1995). US9 was originally described as a member of the US6 family of proteins, which includes the products of the US6-US11 genes. These proteins contain markedly hydrophobic sequences at both the N-and C-termini, and were predicted to insert into cellular membranes (Weston & Barrell, 1986). US9 intracellular localization was initially examined in nonpolarized Madin-Darby canine kidney cells constitutively expressing a C-terminal-tagged ...