Down-regulation of WRN decreases the efficiency of end joining and repair using DNA sequence microhomologies but increases large deletions in misrepaired plasmids. An in vivo LacZα plasmid reactivation assay was used to measure end joining in siRNA down-regulated WRN K562 cells compared with that using siRNA controls. (A) Relative percentage of colonies indicating the efficiency of end joining. (B) The percentage of misrepair, that is, the percentage of the white colonies as a percentage of total colonies (blue + white). (C) The percentage of large deletions, defined as more than 20 bp. (D) Agarose gel showing PCR products of repaired colonies. (E) The percentage of plasmids repaired using DNA sequence microhomologies of 1 to 6 bp. Fifteen plasmids were sequenced. Values significantly different are marked with an asterisk (Student t test; P < .01 for panel A and P < .001 for panels C and E). Error bars reflect the standard error of the mean.