A 3xHA-tagged human SOX9 cDNA sequence and floxed neo were inserted upstream of exon 2 by homologous recombination. Transient cre-mediated recombination excised the neo, leaving only the human SOX9 insertion sequence. Northern blot of rib cartilage in heterozygous mutant mice revealed knocked-in SOX9 gene expression was ~20% of that of endogenous Sox9 gene expression. (J:90567)