Regulation of the ability of STAT3-C to enhance CRU expansion in mice that underwent transplantation. (A) Transplant dose-response effect of the ability of STAT3-C to enhance the in vivo expansion of transplanted HSCs. Different numbers of Stat3-C–transduced bone marrow cells from 5-FU–pretreated Pep3b mice were transplanted into groups of B6 recipients, and then 8 weeks later the marrows of these mice were assayed in secondary mice to determine the numbers of GFP⁺ CRUs that had been regenerated in the primary mice. Primary mice were estimated to have received 1 (1x), 5 (5x), or 50 (50x) GFP⁺ CRUs based on results of other experiments. Values shown are the mean ± 2 SEM for the total number of regenerated Stat3-C–transduced CRUs per primary mouse. The hatched area indicates the total number of CRUs present in a normal B6 mouse. (B) Effects of different mutations of the Ser⁷²⁷ residue of STAT3-C on the repopulating activity of transduced CRUs. 5-FU–enriched bone marrow cells (10⁵) were transduced with MIG, Stat3-C, or a mutant STAT3-C containing either an Asp (CSD) or Ala (CSA) residue instead of Ser⁷²⁷. The cells were then transplanted into irradiated recipients, and the proportion of GFP⁺ WBCs was determined at the times shown. Values shown are the mean ± SEM from 2 independent experiments (5 mice per group for each experiment). The average gene-transfer efficiencies in these experiments were 80%, 73%, 73%, and 76%, respectively, for MIG, Stat3-C, CSA, and CSD.