A floxed PGK-betageo selection cassette was inserted into position -360 of the 5' UTR of the gene. In addition, serine codons 63 and 73 were changed to alanines (S63A and S73A) by site directed PCR-based mutagenesis. In this allele the floxed PGK-betageo cassette was removed by pronuclear injection of a cre-expressing plasmid.