Exon 3 was replaced with a minigene containing Rhoa exons 3 to 6 flanked by loxP sites followed by a neomycin selection cassette and a mutant exon 3 encoding a glycine to valine substitution at amino acid 17. The selection cassette was removed via Flp-mediated recombination. In the absence of cre recombinase, exon 2 is spliced into exons 3-6 minigene resulting in expression of a wild-type mRNA. In the presence of cre recombinase, the floxed exon 3-6 minigene is removed and the mutant G17V allele is expressed.