The +18 region can boost the activity of the Tal1 +19 stem cell enhancer. (A) Schematic representation of the Hprt locus in the HM-1 ES cell line, which carries a deletion of exons 1 and 2. SV40 lacZ reporter constructs with or without the 1.1-kb +18/+19 fragment or the +19 element were cloned into an Hprt targeting vector containing the promoter and exon 1 of the human HPRT locus and 2.9 kb of the mouse genomic locus including exon 2. The targeted Hprt locus contains the SV40 lacZ reporter cassettes integrated upstream of the promoter of the fully functional Hprt locus. (B) ES cells expressing either the SVLac control, SVLac/+19, or SVLac/+18,+19 were plated into differentiation assays and analyzed at days 0, 3, and 8 of differentiation. lacZ activity was assessed by staining with X-gal for 2 h.
