Enhancer D is required for normal transcription at the Ikzf1 locus (A) Diagram of the Ikzf1 BAC transgenic reporter constructs. The first translated exon (exon 2) in the Ikzf1 BAC clone was replaced by the human CD2 (hCD2) reporter gene (white rectangle) inserted at the initiation codon (Ik-BAC-CD2). Two flanking loxP sites (white triangles) were inserted 5′ and 3′ of the D region (Ik-BAC-lDl). The D region was deleted from Ik-BAC-lDl by Cre recombinase to generate Ik-BAC-ΔD construct. (B) Reporter expression in the PBL from Ik-BAC transgenic lines with intact enhancer D region. (C) Reporter expression in PBL from Ik-BAC-ΔD transgenic lines. The copy number is noted besides each founder generated from either the Ik-BAC-CD2 or the Ik-BAC-DL line. The percentage of hCD2⁺ myeloid cells (gray bars), T cells (black bars), and B cells (white bars) was determined for each founder animal by flow cytometry with antibodies to Mac-1, B220, and TCRβ, respectively. N.D., not determined.