A bacterial lacZ gene was inserted into the gene such that the endogenous gene promoter drives expression of beta-galactosidase. RT-PCR analysis detects a high level of gene transcript in the brainstem and olfactory bulb and lower levels in all other tissues analyzed. Beta-galactosidase expression from this allele is detectable in brain, dorsal horns and dorsal root ganglia of the spinal cord and lens of the eye. Expression was also detectable in aorta, a few cells of the heart and kidney, some blood vessels of the lung and liver and tunica albuginea of testis.