Cell type specificity of the Ikzf1 enhancers. (A) Diagrams of the reporter constructs used in this study are shown on the left. Patterns of lineage-specific GFP expression in PB⁻ B cells (B220⁺, B), T cells (TCRβ⁺, T), and myeloid (Mac-1⁺, My) cells are summarized on the right. (B) Ikzf1 enhancer activity in T cells. Error bars (standard deviation) indicate variegation of GFP expression among the different founder lines generated by each enhancer-based reporter. The significance in the difference of GFP expression among GFP-reporter lines was assessed by Student t test; *P < 5 × 10⁻³. (C) Enhancer D and H activity in the HSC/MPP-enriched LSK (Lin^–c-Kit⁺Sca-1⁺), as determined by expression of B-p-GFP-D and -H reporters compared with the parental B-p-GFP reporter. The LMPP is defined as LSK Flt3⁺ (square gate). GFP expression in the LSK is shown as a histogram or together with Flt3 expression as a contour plot. Thin gray line histogram represents a reporter negative control.