Telomerase activity of CD34⁺ cells in liquid culture. (A) Telomerase activity was calculated from the TRAP signal of CD34⁺ cells from all sources (PB, n = 20; CB, n = 11; BM, n = 5; FL, n = 1) at baseline and weeks 1 to 4 of ex vivo expansion culture. Telomerase is expressed as a percentage of the activity detected in the neuroblastoma (SK-N-SH) cell line control. Telomerase was significantly upregulated in week 1 of ex vivo expanded cytokine-supported (KL, IL-3, IL-6, Epo + G-CSF = K36EG) CD34⁺ cells from PB, CB, BM, and FL compared with baseline levels (P < .001). After 2 weeks of expansion, telomerase activity declined decreasing to baseline or below detection after 3 to 4 weeks. (B) Representative TRAP of the regulation of telomerase activity (this example corresponds to a PB sample) of CD34⁺ cells at baseline and weeks 1 through 4 in ex vivo expansion culture. (C) Daily time course experiments in PB CD34⁺ cells in culture (n = 7) showed that after 48 to 72 hours of ex vivo expansion, telomerase was detectable reaching maximum levels between days 5 and 7. (D) Representative TRAP showing telomerase upregulation within days 2 and 3 and highest activity at days 5 and 7 in cytokine-supported (K36EG) CD34⁺ cells in ex vivo expansion culture.