A) Telomerase activity in MCF7 cells is affected by wild-type PML and PML C/C-. (i) Telomerase activity in MCF7 PML STC7 and PML C/C- STC7 cells are increased compared to the MCF7 PCI Neo STC1 cells. (ii) Telomerase activity was reduced in MCF7 PML STC10 while PML C/C- STC22 demonstrated increased activity. Telomerase activity was measured using the TRAPezeXL Telomerase Detection kit according to manufacturer’s protocol (Millipore). Telomerase activity is represented in terms of percentage of the total product generated (TPG) as measured by the assay. B) MCF7 clones demonstrating extremely long telomeres (Figure 5D i) exhibited enhanced susceptibility towards doxorubicin. (i) There were no significant differences in the susceptibility of MCF7 PML STC7 and PML C/C- STC7 towards doxorubicin or tamoxifen. (ii) MCF7 PML STC10 and PML C/C- STC22 demonstrated an increased sensitivity toward doxorubicin. Cell viability was measured using the crystal violet assay. Cells were treated with either 1 μM of doxorubicin or 10 μM of tamoxifen for 24 h. DMSO was used as a vehicle control. At least three independent experiments were performed. *** indicates p < 0.01.