Expansion of Sirt1-deficient HSPCs is dependent on hematopoietic stress. (A) Cell numbers are from two femurs and two tibias of untreated control mice (n = 5), 4-OHT–treated Sirt1-E4^fl/fl, ERT2-Cre (SIRT1^Δ/Δ, n = 3), and 4-OHT–treated ERT2-Cre controls (Cre, n = 3). Error bars represent SD. (B) Before BM transfer, WT and SIRT1^Δ/Δ mice were treated with 4-OHT for 4 wk. After transfer, mice were kept on a 4-OHT–free diet for 20 wk. Sirt1 exon 4 deletion in BM chimeric mice was validated as described in Fig. 1 A (not depicted). At least five recipient mice were analyzed per group for indicated LSK subsets by FACS, and donor origin was confirmed using CD45.2 staining (>95%). Total BM cell numbers from two hind legs are shown. Error bars represent SD. (C) BM from 4-OHT–treated Sirt1-E4^fl/fl, ERT2-Cre mice (SIRT1^Δ/Δ, n = 4) and age-matched 4-OHT–treated ERT2-Cre controls (Cre, n = 5) was mixed with congenic CD45.1⁺ host BM at a 1:1 ratio. The frequency of CD45.2⁺ donor LSK in total BM was analyzed by FACS 20 wk after transfer, and diamonds depict individual recipient mice. Horizontal bars represent the mean of all data points. **, P ≤ 0.01.