The 5' portion of the cytochrome P-450 IIC8 gene has been isolated from a human genomic library. A 3.5 kb genomic fragment including 2477 bp of 5' flanking region and exon 1 of the gene was sequenced. S1 nuclease mapping and specific primer extension experiments localized the transcription start site 23 bp upstream from the ATG. Two and three putative TATA and CAAT boxes were identified together with seven core motifs for glucocorticoid responsive elements, an interesting feature in this constitutive P-450 subfamily.