An 18 kb region between Igl-V1 and Igl-J1 was replaced with a pre-arranged VJ1 gene and a floxed ACN cassette containing a neo resistance gene and sperm-specific cre gene. The ACN cassette is excised in chimeric animals during spermatogenesis. Semiquantitative RT-PCR analysis confirmed the expression of the prearranged VJ1 insertion in splenic B cells. (J:83289)