A bacterial lacZ gene was inserted into the gene such that the endogenous gene promoter drives expression of beta-galactosidase. While RT-PCR analysis detects a gene transcript in many of the tissues analyzed except for lung, liver, kidney, bone marrow, and uterus, beta-galactosidase expression from this allele is detectable in brain, urinary bladder, trachea, larynx, pharynx, tongue, male reproductive system and neuronal ganglia.