Prostacyclin is a potent vasodilator and inhibitor of platelet aggregation. Prostacyclin synthase (PGIS), which converts prostaglandin H2 to prostacyclin, is a member of the cytochrome P450 superfamily. We report here the characterization of the human PGIS gene. The gene is about 60 kb long and contains 10 exons. The PGIS gene was assigned to chromosome 20 by analysis of a human-hamster somatic hybrid DNA panel. The transcription start site was identified by primer extension, and approximately 3 kb of DNA upstream of this site was sequenced. The region surrounding the transcription start site presents features characteristic of a house-keeping gene promoter. It lacks a TATA box but is very rich in GC content. Several putative potential binding sites for transcription factors were identified.