The I isoform-specific translation initiation site in exon 2 was replaced with a stop codon (TGA) and a floxed neomycin resistance cassette inserted downstream of exon 2. RT-PCR analysis of E18.5 calvarial cells confirmed that expression of I isoform is severely reduced by 84% whereas expression of II isoforms is largely retained (>40%) in homozygous mutant mice. Total Runx2 expression is reduced by 67%. (J:223410)