HSCs are maintained in Id1^−/− mice. (A) The percentages and absolute numbers of LSK, CD34loLSK, CMP, MEP, and GMP were determined by flow cytometry of BMC obtained from 12- to 14-week-old Id1^−/− (n = 5) and Id1^+/+ (n = 6) mice in 3 separate experiments (*P < .05). The percentage of each population among total Lin⁻IL-7Rα⁻ BMC (top panel), and the absolute counts to total Lin⁻IL-7Rα⁻ BMC of each group of mice (bottom panel) is shown. (B) Myeloid colony assays were performed on BM, spleen, and PB cells from Id1^−/− and Id1^+/+ mice. BMCs were cultured in methylcellulose semisolid media with cytokines (mouse GM-CSF, 100 ng/mL; mouse IL-3, 30 ng/mL; mouse SCF, 100 ng/mL). The results depict data from triplicate plates (*P ≤ .001). (C) Mice were irradiated (n = 10 per group) and then serially transplanted with 2 × 10⁶ Id1^−/− or Id1^+/+ BMC. The percentage of donor BMC were determined in recipient mice 70 days after transplantation by the use of antibodies specific for donor and host cells (Ly5.1 and Ly5.2). Survival data of recipient mice were plotted as Kaplan-Meier curves and analyzed by log rank test (P = .13). (D) The percentage of donor-derived BMC versus the serial transplant number was plotted (P = .30). Data are representative of 1 of 2 independent experiments. (E) BMC (10⁶) from Id1^−/− or Id1^+/+ mice were mixed with BM competitor cells (10⁶) and were transplanted into irradiated recipient mice. Donor cell chimerism in recipient BMs was evaluated 4 months after transplantation. Left panel graph shows competitive repopulation unit (CRU) of 10⁶ BMC of each group (*P = .03), whereas right panel graph shows CRU data that was normalized by use of the mean total BMC for each group (P = .15). The data are presented as the mean ± SEM (n = 6).