Figure 1. The structure of Cd4 and Cd8 loci, illustrating important cis-acting elements.
a | The expression of CD4 is driven by the Cd4 proximal enhancer in all T-cell populations and is restricted to the double-positive and CD4 single-positive thymocyte populations owing to the activity of the Cd4 silencer, which is found in intron 1. The binding of RUNX (runt-related transcription factor) proteins to the Cd4 silencer is necessary for silencer activity. b | The expression of Cd8ab is driven by a series of stage-specific enhancers (known as E8I – E8IV), which have overlapping activity.