A floxed PGK/Tn5 neomycin resistance (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 9 in the anti-sense direction. Cre-mediated recombination removed the neo cassette. Western blot analysis confirmed increased protein expression in the spleen and thymus. (J:199340)