The percentage of β-gal-negative neurons and β-gal-positive neurons is shown as a percentage of total GABAergic neurons counted from cerebral cortex, not including layer I and white matter tract. β-Gal status was determined as described in Figure 4 and Materials and Methods. The percentage of β-gal-negative neurons is low in the essentially wild-type chimeras 423B and 423C (see Table 1). In contrast, in higher mutant chimeras 423A and 423D, the number of β-gal-negative neurons increases dramatically. This indicates thatcdk5^−/− neurons are contributing significantly to the GABAergic population of the cerebral cortex.