4-Hydroxyphenylpyruvic acid dioxygenase (HPD) is an important enzyme in tyrosine catabolism in most organisms. The activity of this enzyme is expressed mainly in the liver and developmentally regulated in mammals, and a genetic deficiency in this enzyme in humans and mice leads to hereditary tyrosinemia type 3. Using human HPD cDNA as a probe, a chromosomal gene related to HPD was isolated from human gene libraries. The human HPD gene is over 30 kb long and is split into 14 exons. The extract size and boundaries of exon blocks were determined, and all of the splice donor and acceptor sites conformed to the GT/AG rule. Analysis of the 5' flanking sequence of the gene suggests that expression of the gene is regulated by hepatocyte-specific and liver-enriched transcription factors, as well as by hormones. These features of the 5' flanking region of the gene are similar to those of other genes that are specifically expressed in hepatocytes and that are developmentally regulated.