Id1/GFP expression in primitive bone marrow. (A) Id1/GFP expression in Lin⁻ marrow. WBM of the indicated genotypes was immunomagnetically depleted of cells expressing an array of lineage markers (“Bone marrow analyses and cell sorting”), then evaluated by flow cytometry for relative GFP expression. Parentheses show average and SEM of Lin⁻ cell fractions included in the indicated gate (n ≥ 12). (B) Id1/GFP expression in LSK marrow from wild-type (top) and Id1^GFP/GFP mice (bottom). Lin⁻ marrow was stained for Sca1 and c-kit expression. Left plots show staining of total Lin⁻ marrow for these markers. Right panels show cells included in the GFP⁺ gate given in panel A. Boxes outline the LSK fraction. (C) Id1/GFP expression in HSC-enriched fractions of Id1^GFP/GFP marrow. Lin⁻Sca1⁺ marrow was subdivided according to immunofluorescent intensity from additional HSC-related markers as indicated. Left plots show total Lin⁻Sca1⁺ marrow, center plots show Lin⁻Sca1⁺Id1/GFP⁻ cells, and right plots show the GFP⁺ subset. Letters above gates correlate with populations listed in panel E. (D) Id1/GFP expression relative to CD150 and CD48 in WBM (top) and LSK marrow (bottom) as labeled. Box outlines the CD150⁺CD48⁻ HSC-inclusive gate. (E) Relative Id1/GFP expression among several LSK marrow subsets. Each bar represents the average percentage of Id1/GFP⁺ cells of the named population. Phenotype definitions correlate to gated regions with the indicated letters in panels C and D. Averages were calculated from at least 3 independent samples. Error bar represents SEM. Populations are arranged in an ascending order for LT-HSC enrichment.