Exon 4 was flanked with loxP sites. A floxed neo-tk cassette was removed in the ES cells during the generation of this allele. Cre mediated recombination removes exon 4 resulting in a frame shift and at most a short, non-functional peptide. Cre mediated recombination in ES of the parent allele (containing the floxed neo-tk and a third loxP site by exon 4) that removed both the neo-tk cassette and exon 4 produced Lims1tm2Ref. (J:99800)