A bacterial lacZ gene was inserted into the gene such that the endogenous gene promoter drives expression of beta-galactosidase. While RNA transcripts are detected in lung, pancreas, gallbladder, adrenal gland, skeletal muscle and reproductive organs, beta-galactosidase expression from this allele is detectable in brain, spinal cord, sciatic nerve, lymph nodes, kidney, urinary bladder, skeletal muscle, skin, testis, ovary and cervix.