A floxed neomycin selection cassette was inserted in intron 1 and an additional loxP site was inserted 3' to exon 7. The resulting targeted ES cells were subjected to transient cre expression. Exons 2 to 7 as well as neomycin selection cassette were deleted. In addition to this allele, transient expression provided recombinant ES cells in which only the selection cassette was deleted. These cells were used to produce Lipetm2Rze.