A gene encoding a human liver carboxylesterase has been isolated and characterized. Analysis of three overlapping genomic lambda clones revealed that the gene spans about 30 kb and is made of 14 exons being 39 to 379 bp in length. The encoded protein is 550 amino acids long and is highly homologous to carboxylesterases of various mammalian species. The transcription start site was determined by 5'-RACE PCR. An additional 900 bp of DNA from the 5' flanking region of the gene was cloned and sequenced in order to elucidate the structure of the promoter. In this sequence several possible binding sites for transcription factors have been identified, but no TATA-box was present. When different parts of the putative promoter region were ligated in front of the luciferase gene and the constructs were transfected into CHO cells, the reporter gene was effectively transcribed, as demonstrated by the expression of enzyme activity.