Defective HSC-supportive function by Rs1 OBCs. Twelve-week-old control (Ctrl) and ColI(2.3)⁺/Rs1⁺ (Rs1) mice (n = 5-12 per group) were used for these analyses. (A) Quantitative RT-PCR analysis of purified OBCs for expression of the indicated HSC-supporting genes. Data are expressed as log₂ fold relative to the average of the Ctrl samples (set to 0). Averages are shown as black bars. (B) Schematic of the coculture experiment of wild-type (WT) and β-actin-GFP C57BL/6-CD45.2 HSCs (500: counting and methylcellulose; 300: transplantation) with or without Ctrl or Rs1 OBCs. (C) Enumeration of CD45⁺ hematopoietic cells per well. (D) Methylcellulose read-out for myeloid CFU activity per well. (E) Competitive transplantations. A 1:1 ratio of freshly isolated HSCs (150 GFP⁺:150 GFP⁻) or mixed wells containing the progeny of HSCs cocultured on Ctrl (GFP⁺) or Rs1 (GFP⁻) OBCs (300 HSC-derived cell equivalent) were transplanted into lethally irradiated C57BL/6-CD45.1 recipient mice together with 300 000 Sca-1–depleted CD45.1 helper BM cells (n = 3-5 per group). Transplanted mice were bled every 4 weeks and analyzed for the percentage of GFP contribution to donor (CD45.2⁺) chimerism in the peripheral blood. Data are mean ± SD. *P ≤ .05.