The hTERT reporter vector reported hTERT expression in hematopoietic progenitor cells via dGFP expression. (A) The hTERT reporter vector allowed barely detectable levels of dGFP expression in the telomerase-negative CB CD34^–CD15/33⁺ cells. (B) In contrast, nearly all K562 cells became dGFP⁺ following the hTERT reporter vector transduction. (C) hTERT expression in K562 cells was assessed using real-time PCR. No significant differences in endogenous hTERT expression were observed between the hTERT reporter–transduced K562 cells compared with the control vector, or the mock-transduced cells at the indicated days after transduction. Data shown are the averages and standard deviations of hTERT mRNA levels from 3 independent experiments. (D) No significant differences in telomerase activity in the K562 cells were observed between the same 3 groups as analyzed in panel C. Data shown are the averages and standard deviations of telomerase activity. (E) A substantial fraction of the CD34⁺ cells expressed dGFP⁺, both under survival conditions (TPO alone; T) and proliferating conditions (TPO, SCF, and FL; TSF). (F) hTERT mRNA expression was assessed using real-time RT-PCR on the indicated cell populations as described in “Materials and methods.” Data shown are the averages and standard deviations of hTERT mRNA levels from 1250 cells from 5 independent experiments. (G) CD34^–CD15/33⁺ and transduced CD34⁺ cells were assessed for telomerase activity using the TRAP assay. Data shown are the averages and standard deviations of telomerase activity of cell extracts from 1000 cells per assay (n = 3). HI indicates heat-inactivated sample; ND, not detectable. For significance tests, paired t tests were used.