C-type lectins are calcium-dependent carbohydrate-binding proteins with a wide range of biological functions, many of which are related to immunity. DC-SIGN (dendritic cell-specific ICAM-grabbing non-integrin, where ICAM is intercellular adhesion molecule) is a recently described mannose-specific C-type lectin expressed by dendritic cells. Dendritic cells are potent antigen-presenting cells capable of activating T-lymphocytes. DC-SIGN, which is expressed by dendritic cells, binds to ICAM-3 on T-lymphocytes, therefore playing an important role in the activation of T-lymphocytes. DC-SIGN can also bind HIV, and the virus may remain bound to DC-SIGN for protracted periods. DC-SIGN may deliver bound HIV to permissive cell types, mediating infection with high efficiency. A closely related C-type lectin, DC-SIGN-related molecule (DC-SIGNR) has also been described. DC-SIGNR is expressed by restricted subsets of endothelial cells, but has similar ICAM-3 and HIV-binding properties to DC-SIGN. This review describes the mapping of DC-SIGN and DC-SIGNR to chromosome 19p13.3 adjacent to the previously described C-type lectin, CD23 [the low-affinity receptor for immunoglobulin E (FcERII)]. The similar genomic organization of these three genes is discussed and consideration is given to the evolutionary duplications that may underlie this arrangement. Both DC-SIGN and DC-SIGNR possess a neck region, made up of multiple repeats, which supports the ligand-binding domain. Consideration is given to the biological reasons underlying the considerable polymorphism in the numbers of repeats in DC-SIGNR, but not DC-SIGN. The expression patterns of both DC-SIGN and DC-SIGNR are discussed in detail, with particular attention to the expression of both molecules in the placenta, which may have implications for the vertical transmission of HIV. Since dendritic cells may be important in determining the phenotype of many immune responses, via effects on T-lymphocytes, the differential expression of DC-SIGN by particular dendritic cell subsets may have important implications for the immunobiological functions of DC-SIGN. Similarly, the expression of DC-SIGNR by very restricted subsets of endothelial cells may give clues to the function of DC-SIGNR. Finally, the role of DC-SIGN in pathology, particularly in infective and neoplastic processes, is discussed, followed by speculation about likely future developments in this field.