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 lung, spleen, thymus, lymph nodes, skin, adrenal gland, small intestine, large intestine and uterus, beta-galactosidase expression from this allele is detectable in spleen, thymus, lymph nodes and testis.