A floxed neo cassette was inserted in intron 3 and the sequence encoding the alternative splicing donor site for the exonic intron in exon 4 was replaced with tandem stop codons (GTG, AGA to TAG, TGA) via homologous recombination. This prevents the expression of the short transcript but allows expression of the longer transcript that encodes two isoforms (deltaFosb and delta2deltaFosb). Cre-mediated recombination removed the neo cassette.