a: Telomerase activity. Telomerase activity was detected by TRAPeze telomerase detection kit (Millipore, Beverly, MA, USA). Thus, radiolabeled telomere adducts were measured in the wild-type U937 control cells as compared to pMTH1-U937 and asCD11b-U937 cells following culture in the absence and in the presence of 5nM TPA for 24 h up to 72 h, respectively. One representative telomerase assay out of three similar ones is presented. b: Proliferation assay. About 2 × 104 cells (either U937, pMTH1-U937 or asCD11b-U937) were incubated in 96-well microtiter plates in the presence or absence of 5nM TPA for up to 72 h. The cells were radiolabelled with about 0.5 μCi/well [3 H] thymidine for 15 h and at the time points indicated, the cells were harvested and the radioactivity was quantified in a β-scintillation counter. The incorporated [3 H] thymidine was calculated as percentage of the appropriately untreated control cells at each time point which were set to 100%. Data represent the mean ± s.d. (n = 10).