V iral protein R (Vpr) is an accessory protein common to all primate lentiviruses whose role in viral replication remains unclear. It is packaged specifically into virions and is associated with the reverse-transcription complex during early steps of infection (1, 2). Although implicated in various virological processes, the major phenotype ascribed to Vpr is the induction of G 2 /M cell cycle arrest in dividing cells (3, 4). The importance of this phenotype in human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) replication has been notoriously difficult to define because of the minor effects of its deletion on viral replication in cell culture (5). However, reversion of inactivating mutations in both primates and humans highlights that Vpr plays an essential role for viral replication in vivo (6). In addition to cell cycle arrest, several SIV Vpr alleles are capable of targeting the host restriction factor SAMHD1 and that function became separated in the sooty mangabey SIV (SIVsm) lineage upon duplication to generate the vpx gene peculiar to this group (7). Importantly, at present HIV-1 Vpr is not known to retain any SAMHD1 counteractivity (7-10).Vpr-mediated cell cycle arrest depends on its localization to nuclear compartments and the ability to hijack a Cullin-4/DDB1/ Roc1 E3 ubiquitin ligase complex through the WD-containing DCAF-1 adaptor (11,12). For HIV-1 Vpr, this leads ultimately to the activation of the ataxia telangiectasia M and Rad3-related (ATR) and Chk1 kinases to prevent cell cycle progression into mitosis (13). Although several putative Vpr targets have been described, the identity of that which triggers this event has until recently remained elusive. The requirement for the ubiquitin-proteasome system and, in particular, K48-ubiquitin linkages supported a notion that the Vpr target is degraded (14). However, Laguette and colleagues recently found that Vpr interacts with the SLX4 complex, members of the Fanconi anemia DNA repair pathway (15). SLX4, also known as Fanconi anemia complementation group P (FANCP), is a large adaptor protein that acts as a scaffold for a heterodimeric structure-specific endonuclease comprised of MUS81 and EME1. This interaction directs this endonuclease and

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