Mammalian members of the cation diffusion facilitator (CDF) and zrt-, irt-like protein (ZIP) families of Zn transporters, initially identified in Saccharomyces cerevisiae and Arabidopsis thalania, have been cloned during the last 8 years and have been classified as families SLC30 and SLC39 respectively. The cloning of human Zn transporters ZnT-like transporter 1 (hZTL1)/ZnT5 (SLC30A5) and hZIP4 (SLC39A4) were major advances in the understanding of the molecular mechanisms of dietary Zn

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... n. Both transporters are localised at the enterocyte apical membrane and are, therefore, potentially of fundamental importance in dietary Zn uptake. hZTL1 mediates Zn uptake when expressed in Xenopus laevis oocytes and hZIP4 is mutated in most cases of the inherited Zn deficiency disease acrodermatitis enteropathica. Localisation of hZTL1/ZnT5 at the apical membrane of the placental syncytiotrophoblast indicates a fundamental role in the transfer of Zn to the foetus. Observations in rodent models indicate that in the intestine increased Zn availability increases expression of Slc30 Zn transporters. Human intestinal Caco-2 cells show a similar response to increasing the Zn 2+ concentration of the nutrient medium in relation to the expression of mRNA corresponding to several Zn transporters and that of ZnT1 (SLC30A1) and hZTL1/ZnT5 proteins. In the human placental cell line JAR, however, expression at the mRNA level of a number of Zn transporters is not modified by Zn availability, whilst ZnT1 and hZTL1/ZnT5 proteins are reduced under Zn-supplemented conditions. These differences between Caco-2 and JAR cells in Zn transporter gene responses to Zn supply may reflect the different extracellular Zn concentrations encountered by the corresponding cell types in vitro. Dietary zinc absorption: Placental zinc transport: SLC30 transporters: SLC39 transporters The redox stability and ability of Zn 2+ to form polyhedral coordination complexes with a variety of ligands confers properties that are exploited widely in biology, both structurally and catalytically. Zn finger and Zn cluster domains occur repeatedly in biological systems as structural motifs in the DNA-binding domains of proteins, including transcription factors and hormone receptor proteins (Coleman, 1992). Zn is found at the active site of enzymes from all six major classes, where ionisation, polarisation or replacement of Zn 2+ -bound water is involved in the catalytic mechanism (McCall et al. 2000) . Zn is an essential dietary micronutrient, and reported UK intakes (10 . 2 and 7 . 4 mg/d for men and women respectively; Office for National Statistics, 2003) are close to the reference nutrient intake (9 . 5 and 7 . 0 mg/d for men and women respectively; Department of Health, 1991), with 4% of both men and women reporting daily intakes below the reference nutrient intake (Office for National Statistics, 2003). These levels of intake indicate that Zn deficiency is not a major health problem in the UK population. However, the lack of a reliable indicator of Zn status (Wood, 2000) makes subclinical deficiency Abbreviations: CDF, cation diffusion facilitator; DMT1, divalent cation transporter 1; hZIP, human zrt-, irt-like protein; hZTL, human ZnT-like transporter; ZIP, Zip, zrt-, irt-like protein.