Fig. 8. Shh expression in the CNS is controlled by multiple regulatory modules. Schematic view of Shh expression in the CNS, color-coded to depict the distinct regulatory elements governing Shh transcription along the AP axis of the mouse neural tube. Hatched patterns in the floor plate of the spinal cord and hindbrain, in the p3 domain of the diencephalon and in the svz of the telencephalon represent the sites of Shh expression regulated by more than one enhancer. Solid patterns in the ventral midbrain, diencephalon and telencephalon represent sites of Shh expression controlled by single regulatory elements. The location of the six CNS enhancers with respect to the Shh transcription start site is also indicated. The arrows indicate the position of the translocation breakpoints (T1, T2, T3, T4) identified in individuals with HPE (Roessler et al., 1997).