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ency in a single patient with a mild clinical phenotype. As a first step in the generation of mice with hyaluronidase deficiency, we have used experimental and bioinformatic approaches to examine the organization of the mouse chromosome 9 region containing, in order, Hyal2, Hyal1, and Hyal3. This region was found to be complex, with Fus2 partially embedded in Hyal3, and Ifrd2 immediately downstream from Hyal3. The Hyal genes were all found to have four exons, and exons 2-4 exhibited the highest sequence conservation. Northern blot analysis demonstrated that the tissue expression profile for Hyal1 was similar in mice and humans, but a greater number of transcripts was detected in mouse tissues. Hyal3 was expressed more broadly in mice compared with humans and again exhibited additional transcripts. Reverse transcription-PCR demonstrated that some of the larger Hyal1 transcripts, seen on the Northern blot, were the result of cotranscription of Hyal1 with downstream genes, Fus2 or Hyal3. In vitro transcription/translation of one of the high abundance bicistronic transcripts produced Hyal 1, suggesting that Hyal 1 could be produced from all of the bicistronic transcripts. Characterization of the region including mouse Hyal1 and Hyal3 revealed complex organization and transcription that must be considered in the development and interpretation of mouse models involving genes in this region.