A floxed PGk/Tn4 neo cassette and the Akv1-99 murine leukemia virus long terminal repeat were inserted into intron 1 via homologous recombination. The viral long terminal repeat was inserted at the proviral integration site before nucleotide 1062 of the GenBank sequence no. L19607. Cre mediated recombination removed the neo cassette. Q-RT-PCR analysis indicated increased mRNA expression in most tissues in heterozygous and homozygous mice. Western blot analysis confirmed increased protein expression.