The locus was disrupted by the insertion of lacZ and a PGK-neo cassette flanked by two loxP sites into the first exon, thereby deleting the first 21 bp of coding sequence. The neomycin cassette was removed by crossing with mice expressing cre recombinase in the germ line. The reporter gene was transcribed as a fusion mRNA composed of the endogenous 5' UTR, lacZ coding region and 3' region of the endogenous locus, and expression was first observed at the distal part of the visceral endoderm in 5.5 dpc embryos.