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 high level of gene transcript in spleen and lymph nodes, and a lower level in lung, pancreas, thymus, bone marrow, skeletal muscle, stomach, small intestine, large intestine, cecum, testis, epididymis, seminal vesicle, coagulating gland and prostate gland, beta-galactosidase expression from this allele is detectable only in spleen and lymph nodes.