Nucleic Acids Research, 2011, Vol. 39, No. 15 6581 Figure 7. The effect of array length and type on recombinant frequency and UFB formation. (A) Ade+ recombinant frequencies and percentage conversion-types in strains with different length lacO arrays and LacI (+) induction. (B) The same as in (A) but with LacI (++) induction. (C) The percentage of dividing cells with UFBs in a strain with a 12xlacO array after 24 h of LacI (++) induction. (D) Ade+ recombinant frequencies and percentage conversion-types in strains with a 10xFR array with and without EBNA1 induction. In all cases error bars are the standard deviations about the mean. the rDNA repeats of both S. cerevisiae and S. pombe appear to be due to cleavage of converging forks by the Slx1–Slx4 structure-speciﬁc endonuclease (48,49). This is proposed to generate one nicked and one broken chroma- tid, the latter being repaired by HR (48). However, the DNA breaks that we observe in the lacO-LacI array are independent of Slx1 and Mus81 (data not shown). Moreover, cleavage by any of the known fork endonucle- ases would occur ‘behind the fork’ meaning that repair could occur by Rad51-dependent sister chromatid recom- bination. Loss of this type of repair should result in an increase in SSA and therefore in the formation of deletion-types (12). However, deleting rad51 has little or no effect on the overall frequency of recombinants suggesting that the DSBs formed at the lacO-LacI array are generally not repairable by sister chromatid recombin- ation. This implies that DNA breakage either occurs in unreplicated DNA or at essentially the same site in both sister chromatids. The UFBs and DSBs that are induced in strains con- taining a lacO-LacI array occur with similar frequency. As DSBs do not beget anaphase bridges in our experimental system, the correlation in their frequency strongly suggests that DNA breakage stems from the problems in segregating the sister chromatids containing the array. In human cells it is thought that UFBs are caused by DNA catenations between sister chromatids or regions where replication is incomplete (18,19). As the lacO-LacI array