(A) Western blot analysis of SL13 cells pretreated with either 10 μM KU55933 or DMSO and then exposed to 10 μM CPT for 4 hr. After immunoblotting with anti-phospho-Kap1 (S824) and anti-Actin antibodies, the blot was stripped and reprobed with anti-phospho-CHK1 (S345). (B) Western blot analysis of SL13 cells pretreated with either increasing concentration of siATM (5 nM, 10nM, 100nM) or DMSO, and then exposed to 10 μM CPT for 4 hr (Top). Relative expression levels of phosphorylated Kap1-S824 normalized to Actin are the following from left to right: 0.03, 1.00, 0.64, 0.46, 0.22. In the bottom blot, a final concentration of 100nM of siATM was used. After immunoblotting with anti-phospho-CHK1 (S345), the blot was stripped and re-probed with anti-Actin antibody. (C) Representative analysis of PacBio CCS reads (maximum of 200 shown for simplicity) from samples pretreated with DMSO, 10 μM KU55933 or 100 nM siATM prior to doxycycline treatment. (D) Bar graph shows percentage of CCS reads with de novo telomere addition. The means and standard error of mean (SEM) from multiple experiments (n) are the following: 19.56 ± 2.06 for DMSO, 12.45 ± 1.16 for KU55933 and 0 ± 0 for siATM. Asterisk indicates unpaired t-test two-tailed P value is <0.05.