Absence of telomerase activity blunts NA10hd-induced self-renewal of myeloid progenitors and HSCs in vitro. Cultures were initiated with WT or Tert^−/− BM cells from 5-FU pre-treated mice. Cells were transduced with NA10hd, expanded for 6 days in vitro and either plated in methylcellulose medium (A) or transplanted into lethally irradiated recipients (B). (A) Each 7 days of methylcellulose culture, generated colonies were counted, cells harvested, pooled and equal cell numbers replated for a total of 3 times to calculate the yield of granulocyte-macrophage colonies formed. Statistical significance was determined by application of the paired Student t test and is shown as *P < .05. (B) Mice were transplanted with limiting dilutions of cells used to initiate the cultures and with cells harvested at the end of expansion in vitro. Proportions of circulating B, T, and myeloid donor-derived (CD45.2⁺ or GFP⁺) WBCs were determined 4-6 months later. Fold in vitro expansions of WT and Tert^−/− HSCs stimulated by NA10hd was estimated by determining the frequency, and hence HSC content in suspensions used to initiate and harvested from in vitro cultures. Results are expressed as the mean ± SEM of 2 independent experiments.