A portion of exon 1 following the ATG codon, exon 2, and intron 1 were replaced with a cassette containing an EGFP reporter gene, the thymidine kinase promoter with a simian virus 40 polyA signal and a PGK-neomycin gene flanked by two loxP sites, creating an out-of-frame deletion. Western blot confirmed absence of protein expression. EGFP expression is driven by the Hsbp1 promoter and is detected at E0.5 and E1.5 in blastocysts and during later stages (E8.5-10.5) in heterozygotes.