Mouse HSCs have a reservoir of telomeres sufficient to sustain their self-renewal during several cycles of serial transplantation. (A) Experimental protocol in which whole bone marrow (WBM) cells from either WT or Tert^−/− mice were transplanted into lethally irradiated primary recipients and their telomere length monitored at various time points post transplantation (PT). (B) The average telomere length of donor-derived WT (blue bars) or Tert^−/− (red bars) BM cells 6 weeks, 3 and 9 months PT within primary recipients. Dotted lines indicate the telomere length of donor WT (blue) or Tert^−/− (red) BM cells on the day of the primary transplantation. Error bars denote SD. N^wt = 5 and N^tert−/− = 5 at 6-week, 3- and 9-month time point analysis. (C) Serial transplantation protocol of either WT or Tert^−/− WBM cells. (D) The average telomere length of donor-derived WT (blue bars) or Tert^−/− (red bars) BM cells measured at the time of each transplantation. Dotted lines indicate the telomere length of donor WT (blue) or Tert^−/− (red) BM cells on the day of the primary transplantation. Error bars denote SD. N^wt = 6; 6; 4 and N^tert−/− = 6; 4; 2 at 3-month PT time point analysis of 1°, 2°, and 3° recipients, respectively. (E) Percent donor reconstitution in PB of primary, secondary and tertiary recipients (generated as described in panel C) transplanted with unmanipulated WT (blue triangles) and Tert^−/− (red triangles) BM cells. Each triangle represents an individual recipient. Each horizontal line represents the mean percent donor reconstitution of at least 3 recipients.