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 spleen and lymph nodes, moderate levels in lung, thymus, bone marrow, gallbladder, pituitary gland, uterus and white fat and low levels in brain, cortex, subcortical region, cerebellum, brainstem, olfactory bulb, spinal cord, Harderian glands, heart, liver, kidney, skin, urinary bladder, adrenal gland, salivary gland, skeletal muscle, tongue, stomach, small intestine, large intestine, cecum, testis, epididymis, coagulating gland, prostate gland and ovaries. However, beta-galactosidase expression from this allele is detectable only in testis with strong staining in several spermatogenic cells in the seminiferous tubules of testis.