Impaired HSC activity due to Cited2 deficiency. Competitive reconstitution was performed by transplanting 10⁶ fetal liver cells from the recipient strain as competitor cells (CD45.1⁺) and 10⁶ fetal liver cells from Cited2^−/− (n = 3), Cited2^+/− (n = 3), and Cited2^+/+ littermate control (n = 3) at 14.5 dpc (CD45.2⁺). Percentage of CD45.2⁺ and CD45.1⁺ cells in the peripheral blood was analyzed and donor chimerism was determined as [%CD45.2⁺/(%CD45.1⁺ + %CD45.2⁺)] × 100. The chimerism data were expressed as average plus or minus SD (below “Donor chimerism”). (A) The gating was performed in 2 steps to exclude the CD45.1 and CD45.2 double-positive cells and artifacts in the analysis. First, viable nucleated cells (R1) according to forward and side scatter characteristics were gated to gain CD45.1 and CD45.2 positivity. Then, CD45.1 and CD45.2 double-positive cells (ranging from 0.95% to 4.5%) were gated out and the R2 was retained for further analysis, which includes UL, LL, and LR quadrants. (B) Representative histograms plotted after gating on R1 and R2. Compared with Cited2^+/+ littermate controls, Cited2^−/− fetal liver cells exhibited severely impaired reconstitution ability reflected by significantly decreased donor chimerism (*P < .01). Cited2^+/− fetal liver cells showed significant reduced reconstitution as well (*P < .01). The percentages above the brackets represent the CD45.2⁺ cells.