A pair of mFRT71 sites have been introduced into the endogenous Wnt4 locus, such that they flank the first exon, resulting in a conditional null allele in which Wnt4 function can be disrupted by using FLPm5 recombinase to excise this artificial mFRT71 cassette. A loxP cassette containing a Avic\GFP3xP3.cUa marker has also been inserted, in the first intron, downstream of the second mFRT71 site.