The human thromboxane A2 gene, present as a single copy, spans over 15 kilobases (kb) and contains 3 exons divided by 2 introns. The first intron, intron 1, exists in the 5'-noncoding region, 83 base pairs upstream from the ATG start site and is 6.3 kb long. Intron 2 with a length of 4.3 kb is located at the end of the sixth transmembrane region, thereby separating it from the downstream coding sequences including the seventh transmembrane region and the 3'-untranslated region. By rapid amplification of 5'-cDNA ends, transcription initiation sites starting in two different putative promoter regions were determined. In the 5'-flanking region of these transcription initiation sites, no typical TATA box exists. The major promoter, the promoter region I, contains four potential SP-1 binding sites and several potential binding sites for other transcription factors. By polymerase chain reaction analysis, a small portion of the RNA transcribed from this gene was shown to contain an additional 115-base pair long noncoding exon, exon 1b, which is located in intron 1. No additional exon in intron 2 was detectable, indicating that a single type of thromboxane A2 receptor protein is encoded by this gene. Chromosomal localization was carried out by fluorescence in situ hybridization of cloned genomic DNA to the metaphase chromosome. The gene was assigned to 19p13.3 of human chromosome.