In general, β-gal staining was substantially less intense in embryos as compared to adults for all four founder lines indicating a developmental effect on β4 promoter activity. In fact, founder lines 30 and 39 showed almost no detectable staining in the central nervous system at ED18.5 but showed extensive staining at 1 month of age (see below). The two other lines (lines 54 and 447) however, did exhibit significant β-gal staining early in development, but again, the staining was more intense in older animals (see below). As shown in Figures 2 - 4, the β4 promoter region directs rather striking cell-type-specific expression of the β-gal gene in ED18.5 transgenic embryos of founder lines 54 and 447. In the central nervous system, there is very strong β-gal staining in the cortex (intermediate and ventricular zones, Fig. 2A), the medulla oblongata (Fig. 2C), the pons (Fig. 2E) and the spinal cord (Fig. 2G, I and K). Less intense staining is seen in other regions of the central nervous system including the piriform cortex, subiculum, lateral and medial habenular nuclei and the pontine
