A loxP site was inserted into intron 1 and a FRT-flanked neomycin resistance gene cassette and a second loxP site into intron 3. The neo cassette and exons 2 and 3 were deleted through subsequent cre-mediated recombination, creating a null allele. Mice carying this allele develop SLE (systemic lupus erythematosus) like phenotypes. (J:238440)