Homologous recombination introduced loxP sites flanking the 2 exons of the gene. Mice carrying the targeted mutation were bred to the HSP-cre deleter strain resulting in deletion of the 2 exons and of half of the 5' upstream CpG islands. Absence of gene expression in yolk sacs of homozygous mutant embryos was demonstrated by Northern blot analysis. (J:76871)