An 8.4 kb region, containing the entire Tnf gene and the last coding exons of the Lta and Ltb genes, was deleted by cre mediated recombination of surrounding loxP sites inserted within Lta and Ltb via homologous recombiantion. Partial coding regions of Lta and Ltb were left intact separtated by a single loxP site, however RT-PCR analysis of homozygous mutant splenocytes activated by concanavalin A indicated an absence of transcript from each locus.