The +18 region does not behave as a classical enhancer. (A) Schematic of the Tal1 locus indicating the positions of the 0.9E3 Tal1 promoter, +18 and +19 regions. (B) Whole-mount staining of mid-gestation transgenic embryos expressing lacZ under the control of either the SV40 minimal promoter or the 0.9E3 Tal1 promoter with and without the +18, the +19, or the +18/+19 elements combined. (C) The +18 region boosts the activity of the Tal1 +19 enhancer in 416B stable transfection assays. Shown on the left are the reporter constructs of the +18 element, the 640-bp +19 element, and the 1.1-kb +18/+19 fragment inserted downstream from the SV40 luciferase reporter cassette. The results are shown on the right with the luciferase values presented as fold increase over the SV40 luciferase control, which was assigned a value of 1.