He transfected cells were cultured for 48 h at 37uC and then the optimum numbers of the cells were plated onto 96-well plates in medium containing 400 mg/mL hygromycin. After 14 days of culture, targeted clones were confirmed by PCR using primers REV3-KO GT-Fw and 59-loxP for knockout and REV3-CD GT-Rv and 39-loxP for the catalytically dead mutant(Table S1). The PCR products for the catalytically dead mutant were digested with NarI to confirm introduction of the mutations in the 39-arm. The elimination of exon 5 in mRNA was confirmed by RT-PCR using primers of REV3 ex1 Fw and ex7 Rv. The presence of the designed mutations in the REV3L gene was demonstrated by RT-PCR using primers of REV3 mRNA Fw and Rv followed by DNA sequencing.Statistical AnalysisStatistical significance was examined by the Student’s t-test. Levels of P,0.05 were considered to be significant.Table 3. Gene targeting efficiency at REV3 loci using targeting vectors for the knockout mutation (knockout) and the catalytically dead mutation (knock-in).Hygromycin resistant colonies Knockout Nalm-6 Nalm-6 MSH+ Knock-in Nalm-6 Nalm-6 MSH+ 68 24 36PCR positive Targeting efficiency NarI positive colonies ( ) coloniesMutation introducing efficiency ( )925????1826913 8.doi:10.1371/journal.pone.0061189.tEstablishment of Human Cell Line Nalm-6-MSH+Figure 9. Establishment of Nalm-6-MSH+. The MSH2-expressing cell lime, 
i.e., Nalm-6-MSH+, has been established by introduction of cDNA of exon 9 to 16 of the MSH2 gene into the original Nalm-6 cells. The resulting cell line exhibits high efficiency of gene targeting as the original Nalm-6 and is genetically stable. It is also resistant to killing effects of alkylating agents. doi:10.1371/journal.pone.0061189.gResults CGH Array Analyses of Nalm-6 GenomeTo explore the cause of deficiency in mismatch repair functions in Nalm-6 cells, we first examined the proteins of MSH2 and MSH6 by the Western blotting analysis and confirmed that MSH2 was not expressed and MSH6 was poorly expressed in Nalm-6 (Fig. 1A) [20]. Next, we examined transcripts of the genes by RTPCR. Although the transcript of the MSH6 gene was detected (data not shown), no RT-PCR product was detected for MSH2 gene (Fig. 1B), which were consistent with the previous report [6]. Then, we surveyed mutations in the genomic DNA of Nalm-6 by the CGH array analysis to identify the cause of defect of MSH2 expression. We found that the chromosome 2 had compound heterologous deletions in the MSH2 gene (Fig. 15755315 2). The whole MSH2 gene was Methyl linolenate chemical information deleted in one allele, while chromosomal JW 74 region from exon 9 to exon 16 was deleted in another allele. These results indicated that the MSH2 gene in Nalm-6 completely lost the region between exon 9 and exon 16.neomycin-resistance gene and the cassette containing splicing acceptor of intron 8, artificial exon combined from exon 9 to exon 16 and 39-UTR region. The neomycin-resistance gene was flanked by two mutant loxP sequences. After the introduction of DNA region from exon 9 to exon 16, the drug resistance gene was cured by transient expression of Cre recombinase. After the gene targeting, the resulting cells transcribed mRNA of the MSH2 gene and expressed MSH2 protein (Fig. 1 A, B). Furthermore, MSH6 protein was clearly detectable in the targeted cells (Fig. 1A). The growth of the cells expressing MSH2/MSH6, i.e., Nalm-6-MSH+, (2160.3 h) was slightly slower than the original Nalm-6 cells (1960.6 h). Spontaneous HPRT gene mutation frequency in Nalm-6-MSH+ (3.661.He transfected cells were cultured for 48 h at 37uC and then the optimum numbers of the cells were plated onto 96-well plates in medium containing 400 mg/mL hygromycin. After 14 days of culture, targeted clones were confirmed by PCR using primers REV3-KO GT-Fw and 59-loxP for knockout and REV3-CD GT-Rv and 39-loxP for the catalytically dead mutant(Table S1). The PCR products for the catalytically dead mutant were digested with NarI to confirm introduction of the mutations in the 39-arm. The elimination of exon 5 in mRNA was confirmed by RT-PCR using primers of REV3 ex1 Fw and ex7 Rv. The presence of the designed mutations in the REV3L gene was demonstrated by RT-PCR using primers of REV3 mRNA Fw and Rv followed by DNA sequencing.Statistical AnalysisStatistical significance was examined by the Student’s t-test. Levels of P,0.05 were considered to be significant.Table 3. Gene targeting efficiency at REV3 loci using targeting vectors for the knockout mutation (knockout) and the catalytically dead mutation (knock-in).Hygromycin resistant colonies Knockout Nalm-6 Nalm-6 MSH+ Knock-in Nalm-6 Nalm-6 MSH+ 68 24 36PCR positive Targeting efficiency NarI positive colonies ( ) coloniesMutation introducing efficiency ( )925????1826913 8.doi:10.1371/journal.pone.0061189.tEstablishment of Human Cell Line Nalm-6-MSH+Figure 9. Establishment of Nalm-6-MSH+. The MSH2-expressing cell lime, i.e., Nalm-6-MSH+, has been established by introduction of cDNA of exon 9 to 16 of the MSH2 gene into the original Nalm-6 cells. The resulting cell line exhibits high efficiency of gene targeting as the original Nalm-6 and is genetically stable. It is also resistant to killing effects of alkylating agents. doi:10.1371/journal.pone.0061189.gResults CGH Array Analyses of Nalm-6 GenomeTo explore the cause of deficiency in mismatch repair functions in Nalm-6 cells, we first examined the proteins of MSH2 and MSH6 by the Western blotting analysis and confirmed that MSH2 was not expressed and MSH6 was poorly expressed in Nalm-6 (Fig. 1A) [20]. Next, we examined transcripts of the genes by RTPCR. Although the transcript of the MSH6 gene was detected (data not shown), no RT-PCR product was detected for MSH2 gene (Fig. 1B), which were consistent with 
the previous report [6]. Then, we surveyed mutations in the genomic DNA of Nalm-6 by the CGH array analysis to identify the cause of defect of MSH2 expression. We found that the chromosome 2 had compound heterologous deletions in the MSH2 gene (Fig. 15755315 2). The whole MSH2 gene was deleted in one allele, while chromosomal region from exon 9 to exon 16 was deleted in another allele. These results indicated that the MSH2 gene in Nalm-6 completely lost the region between exon 9 and exon 16.neomycin-resistance gene and the cassette containing splicing acceptor of intron 8, artificial exon combined from exon 9 to exon 16 and 39-UTR region. The neomycin-resistance gene was flanked by two mutant loxP sequences. After the introduction of DNA region from exon 9 to exon 16, the drug resistance gene was cured by transient expression of Cre recombinase. After the gene targeting, the resulting cells transcribed mRNA of the MSH2 gene and expressed MSH2 protein (Fig. 1 A, B). Furthermore, MSH6 protein was clearly detectable in the targeted cells (Fig. 1A). The growth of the cells expressing MSH2/MSH6, i.e., Nalm-6-MSH+, (2160.3 h) was slightly slower than the original Nalm-6 cells (1960.6 h). Spontaneous HPRT gene mutation frequency in Nalm-6-MSH+ (3.661.