The structure and organization of mouse hyaluronan synthase 1 gene, HAS1 were determined by direct sequencing of lambda phage clones carrying the entire gene and by application of the long and accurate (LA)-PCR method to amplify regions encompassing the exon-intron boundaries and all of the exons. This gene spans about 11kb of genomic DNA and consists of 5 exons and 4 introns. A similarity in the exon-intron organization was found between the genes of mouse HAS1 and Xenopus laevis DG42 which was recently identified as Xenopus hyaluronan synthase. The transcription initiation site was determined by rapid amplification of the cDNA ends (5'-RACE). Position +1 is located 55 nucleotides upstream of the ATG initiation codon. The promoter region of the HAS1 gene has no typical TATA box, but contains a CCAAT box located 190 nucleotides upstream of the transcription initiation site. Further analysis of 1.4 kb of the 5' flanking region revealed several potential binding motifs for transcription factors. This information about the gene structure may be useful for further studies on the promoter activity.