Exon 3 was replaced with a LacZ reporter cassette. The lacZ cassette encoded the beta-gal coding region preceded by stop codons (TAG) in all three reading frames. The construct also uses a polyA sequence to replace the untranslated region of the gene locus, which contains AU-rich instability sequences. Beta-gal activity was detected in numerous tissues of heterozygous embryos including the mammary bud, inner ear, nasal epithelium, and in the endochondral skeleton. Beta-gal activity was also observed in tendon insertion sites of adult mice and within descrete populations of chondrocytes.