The basic helix-loop-helix gene Math1, a positive regulator of neuronal differentiation, is specifically expressed in the dorsal part of the developing nervous system. To determine the effects of ectopic expression of Math1, we generated two transgenic mouse lines; One carried the Cre recombinase gene under the control of the nestin promoter and enhancer, which direct expression in neural precursor cells, and the other carried the Math1 gene, the expression of which was regulated by the cytomegalovirus (CMV) promoter but interrupted by the stop cassette flanked by loxP sites. In F1 embryos that carried the two transgenes, the stop cassette was removed by Cre recombinase in the developing nervous system, and Math1 expression was ectopically directed from the CMV promoter. We found that these embryos exhibited abnormal morphology of the brain and extensive cell death in the nervous system. These results suggest that ectopic expression of Math1 is toxic to neurons and leads to apoptosis.