The Zeta class of cytosolic glutathione-S-transferases (GSTs) has recently been identified and spans a range of species from plants to humans. The cDNA and protein of a human member of this class have been previously characterised in our laboratory. This cDNA has also been described as maleylacetoacetate isomerase (MAAI), an enzyme of the phenylalanine catabolism pathway (Fernandez-Canon and Penalva, 1998). The present study has determined the structure and chromosome location of the gene encoding human GSTZ1/MAAI. The gene spans approximately 10.9 kb and is composed of 9 exons. Three intron positions of GSTZ1 were precisely conserved compared to the carnation and Caenorhabditis elegans Zeta GST genes. Fluorescent in situ hybridization mapped the gene to a single locus on chromosome 14q24.3, which is in agreement with an independent localization between the Genethon markers D14S263 and D14S67. The coding region of the gene differed from the GSTZ1 cDNA at two nucleotide positions in exon 3, resulting in Lys-32-->Glu and Arg-42--> Gly substitutions. This gene structure information will allow analysis of the polymorphism in genomic DNA samples, and enables further investigations into genetic defects in this step of the phenylalanine catabolism pathway.