Exon 2 was replaced with loxP-flanked cDNA for exons 2 to 4, with splice acceptor site at the 5' end and a polyA signal at the 3' end. Cre recombinase removed the loxP-flanked sequence, leaving behind a loxP site and downstream artificial exon with a splice acceptor, Flag epitope coding sequence, stop codon and c-fos sequence. Western blot analysis confirmed the absence of wild-type protein in testis lysates from homozygous mice. (J:204433)