following one. Since PCNA is sumoylated even in the absence of any treatment during S phase (31) and since the same lysine is the ubiquitin target of Rad18/Rad6, it is reasonable to believe that the sumoylated PCNA drives replication and avoids unintended mutation and recombi- nation events to take place. On one hand, sumoylation of PCNA on the lysine 164 would protect against unneces- sary ubiquitination and, consequently, hinder the error- prone and error-free Rad18-dependent pathways from acting. Notably, it would prevent a recruitment of an error prone polymerase that could introduce untar- geted mutations. On the other hand, the sumoylated PCNA would recruit Srs2 that counteracts recombina- tional repair. When the replicative polymerase encounters a blocking lesion, a single-stranded region upstream of the synthesis block is formed, due to the uncoupling of the two nascent strands synthesis upon fork progression (33). Desumoylation of PCNA through a speciﬁc isopeptidase would occur, or new unmodiﬁed PCNA monomers would replace the sumoylated PCNA trimers. The absence of sumoylated PCNA opens two nonexclusive possibilities: ubiquitination of PCNA and formation of a Rad51 nucleo- protein ﬁlament on the ssDNA region. Fork restoration could then be mediated through the Rad18 pathways or by recombination. It is important to note that the choice of one or the other pathway is likely not determined by the ubiquitination state of PCNA: it is known that recombina- tional repair plays an important role in cells unable to ubiquitinate PCNA (e.g. rad18 or more evidently rad18 srs2 mutants) and in cells where PCNA is presumably ubiquitinated (e.g. rev3 or rad5 cells). Therefore, a possibil- ity is that both PCNA ubiquitination and formation of a Rad51 nucleoprotein ﬁlament occur concomitantly. If fork restoration occurs by translesion synthesis, the Rad51 nucleoprotein ﬁlament might be eliminated upon progres- sion of the polymerase. In this scheme where PCNA is ubiquitinated upon repli- cation arrest, the single siz1 mutation is not expected to have any eﬀects on survival to genotoxic treatments, nor on induced mutation or recombination frequencies. The only siz1 eﬀects that may be predicted in otherwise wild- type cells is an increase of untargeted mutations or recom- bination events, due to inadvertent switch of polymerases or/and Rad51 binding. However, these eﬀects could be too small to overcome the other sources of spontaneous muta- tion or recombination and to be easily detectable. ACKNOWLEDGEMENTS We are grateful to Marco Foiani and Martine Heude for providing strains and antibodies. We also thank Pablo Radicella, Ste´ phanie Marsin, Serge Boiteux and Eric Coı¨ c for helpful discussions and technical advices. This study was supported by grants of the Institut National du Cancer (PL003), the European Community (C.L.B., S.G., F.F., X.V., LSHG-CT-2003-503303), the Agence Nationale de la Recherche (ANR-07-BLAN-0350-01) and fellowships of La Ligue contre le Cancer (P.D.) and the Association pour la Recherche sur le Cancer (C.L.B.), Nucleic Acids Research, 2008, Vol. 36, No. 15 4973 the Centre National de la Recherche Scientiﬁque and the Commissariat a` l’Energie Atomique. Funding to pay the Open Access publication charges for this article was pro- vided by Commissariat a` l’Energie Atomique. Conﬂict of interest statement. None declared. REFERENCES 1. Singleton,M.R., Dillingham,M.S. and Wigley,D.B. (2007) Structure and Mechanism of Helicases and Nucleic Acid Translocases. Annu. Rev. Biochem., 76, 23–50. 2. Wu,L. and Hickson,I.D. (2006) DNA helicases required for homologous recombination and repair of damaged replication forks. Annu. Rev. Genet., 40, 279–306. 3. 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