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Exon 1 was flanked by loxP sites and subsequently excised via in vitro cre mediated recombination. The resultant germline allele contained a single loxP site in place of exon 1, which included the 5' untranslated region and encoded the start codon, signal peptide, and part of the extracellular domain. RT-PCR indicated an absence of transcript in homozygous mutant embryos. (J:80043)