Irinotecan, a drug widely used in the treatment of advanced colorectal cancers, is a prodrug requiring activation to 7-ethyl-10-hydroxycamptothecin (SN-38) by carboxylesterase 2 (hCE2). The existence of functional polymorphisms in the gene encoding this enzyme could explain the individual variability in drug efficacy and toxicity. We have explored this possibility in looking for single nucleotide polymorphisms and their functional consequence.In a series of 115 human deoxyribonucleic acid samples, we have explored the 12 exons of the hCE2 gene, the intron-exon junctions, and the 5'- and 3'-untranslated regions, by denaturing HPLC and sequencing of polymerase chain reaction products. The functionality of the variations identified was studied in 60 human liver samples by measuring hCE2 gene expression by real-time reverse transcriptase-polymerase chain reaction of messenger ribonucleic acid extracts and carboxylesterase activity by use of irinotecan as a substrate.We have identified a total of 11 single nucleotide polymorphisms, none of them able to alter the amino acid sequence of the protein. They are distributed in 10 distinct genotypes in addition to the wild type. The most frequent variation (localized in IVS10) has an allele frequency of 0.17 and has been identified at the homozygous state in 1 sample. hCE2 gene expression and carboxylesterase activity in the variants identified were not significantly different from those measured in wild-type samples.The hCE2 gene presents several polymorphisms, none of which seems to be involved in significant variations in protein activity and, therefore, in irinotecan activation.