Osome ends carry telomeres which protect the ends from being recognized as double-strand Chebulagic acid breaks [33]; therefore the lack of telomere 3-Amino-1-propanesulfonic acid site signals at the broken or un-rejoined ends would indicate de novo breaks. Centromeric regions were identified by pan-centromere FISH, as well as by the intense DAPI staining and sister chromatid constrictions. We found that structural chromosome aberrations were exclusively non-clonal in these cell lines at early population doublings (PD 14 -15) (Table S1). Surprisingly, the majority (68 ) of non-clonal aberrations (four cell lines pooled) occurred in pericentromeric or centromeric regions (band p11?q11). The four cell lines were followed for chromosome aberration analysis at later PDs when clonal structural aberrations were observed in each cell line (Table S1 and Table S2). The most common breakpoints in those clonal aberrations were again in the pericentromeric or centromeric regions (underlined in Table S2 and indicated by arrows in Figures S1 and S2). Moreover, similar fractions of non-clonal pericentromeric aberrations including de novo pericentromeric deletions were detected at the later PDs as compared with earlier PDs (Table S1). Those de novo pericentromeric deletions were confirmed by the absence of telomere signals at the deleted pericentromeric regions as exemplified in Figure 1. Thus, from the results of 
clonal pericentromeric aberrations in advanced PDs and the persistent occurrence of non-clonal pericentromeric aberrations in all four cell lines, we concluded that epithelial cells expressing HPV16 E6E7 and hTERT had intrinsic pericentromeric instability.Pericentromeric Regions Exhibited Instability Induced by Aphidicolin Treatment in Cells Expressing HPV16 E6E7 and hTERTIt is intriguing that the structural chromosomal instability in cells co-expressing HPV16 E6E7 and hTERT cells occurred preferentially in the pericentromeric or centromeric regions. It has been speculated that the condensed structure of pericentromeric or centromeric heterochromatin can present barriers to DNA replication or result in problematic progression of replication fork. Therefore pericentromeric regions, like other known fragile sites, are expected to be hotspots of DNA lesions under 1081537 replication stress. Aphidicolin (APH), a reversible inhibitor of eukaryotic DNA polymerases a and e, is a classical drug used for inducing instability at chromosomal fragile sites when applied at low doses that partially inhibit replication fork progression [19,22]. We therefore investigated whether pericentromeric regions exhibited instability under replication stress. The four HPV16 E6E7-hTERT-expressing cell lines at PD 80 were treated with 0.6 mg/ml of APH and vehicle (0.1 DMSO) for 24 h, and harvested at the end of treatment. For each cell line, 100 metaphases were analyzed for chromosome aberrations using SKY. We observed a dramatic increase in the frequencies of chromatid breaks in all four cell lines under APH treatment (P#0.05) (Figure 2A). Most of chromatid breaks were located at known non-centromeric fragile sites [20] as exemplified in Figure 2B (upper panel). Chromatid breaks in pericentromeric regions (exemplified in Figure 2B, lower panel) accounted for about 20 of total chromatid breaks (Figure 2A). These results demonstrated that pericentromeric regions in HPV16 E6E7hTERT-expressing cells resembled fragile sites that exhibited instability under APH-induced replication stress, yet the APHinduced instability did not pred.Osome ends carry telomeres which protect the ends from being recognized as double-strand breaks [33]; therefore the lack of telomere signals at the broken or un-rejoined ends would indicate de novo breaks. Centromeric regions were identified by pan-centromere FISH, as well as by the intense DAPI staining and sister chromatid constrictions. We found that structural chromosome aberrations were exclusively non-clonal in these cell lines at early population doublings (PD 14 -15) (Table S1). Surprisingly, the majority (68 ) of non-clonal aberrations (four cell lines pooled) occurred in pericentromeric or centromeric regions (band p11?q11). The four cell lines were followed for chromosome aberration analysis at later PDs when clonal structural aberrations were observed in each cell line (Table S1 and Table S2). The most common breakpoints in those clonal aberrations were again in the pericentromeric or centromeric regions (underlined in Table S2 and indicated by arrows in Figures S1 and S2). Moreover, similar fractions of non-clonal pericentromeric aberrations including de novo pericentromeric deletions were detected at the later PDs as compared with earlier PDs (Table S1). Those de novo pericentromeric deletions were confirmed by the absence of telomere signals at the deleted pericentromeric regions as exemplified in Figure 1. Thus, from the results of clonal pericentromeric aberrations in advanced PDs and the persistent occurrence of non-clonal pericentromeric aberrations in all four cell lines, we concluded that epithelial cells expressing HPV16 E6E7 and hTERT had intrinsic pericentromeric instability.Pericentromeric Regions Exhibited Instability Induced by Aphidicolin Treatment in Cells Expressing HPV16 E6E7 and hTERTIt is intriguing that 
the structural chromosomal instability in cells co-expressing HPV16 E6E7 and hTERT cells occurred preferentially in the pericentromeric or centromeric regions. It has been speculated that the condensed structure of pericentromeric or centromeric heterochromatin can present barriers to DNA replication or result in problematic progression of replication fork. Therefore pericentromeric regions, like other known fragile sites, are expected to be hotspots of DNA lesions under 1081537 replication stress. Aphidicolin (APH), a reversible inhibitor of eukaryotic DNA polymerases a and e, is a classical drug used for inducing instability at chromosomal fragile sites when applied at low doses that partially inhibit replication fork progression [19,22]. We therefore investigated whether pericentromeric regions exhibited instability under replication stress. The four HPV16 E6E7-hTERT-expressing cell lines at PD 80 were treated with 0.6 mg/ml of APH and vehicle (0.1 DMSO) for 24 h, and harvested at the end of treatment. For each cell line, 100 metaphases were analyzed for chromosome aberrations using SKY. We observed a dramatic increase in the frequencies of chromatid breaks in all four cell lines under APH treatment (P#0.05) (Figure 2A). Most of chromatid breaks were located at known non-centromeric fragile sites [20] as exemplified in Figure 2B (upper panel). Chromatid breaks in pericentromeric regions (exemplified in Figure 2B, lower panel) accounted for about 20 of total chromatid breaks (Figure 2A). These results demonstrated that pericentromeric regions in HPV16 E6E7hTERT-expressing cells resembled fragile sites that exhibited instability under APH-induced replication stress, yet the APHinduced instability did not pred.