Prominin is the first identified member of a novel family of polytopic membrane proteins conserved throughout the animal kingdom. It has an unusual membrane topology, containing five transmembrane domains and two large glycosylated extracellular loops. In mammals, prominin is expressed in various embryonic and adult epithelial cells, as well as in nonepithelial cells, such as hematopoietic stem cells. At the subcellular level, prominin is selectively localized in microvilli and other plasma membrane protrusions, irrespective of cell type. At the molecular level, prominin specifically interacts with membrane cholesterol and is a marker of a novel type of cholesterol-based lipid 'raft'. A frameshift mutation in the human prominin gene, which results in a truncated protein that is no longer transported to the cell surface, is associated with retinal degeneration. Given that prominin is concentrated in the plasma membrane evaginations at the base of the outer segment of rod photoreceptor cells, which are essential precursor structures in the biogenesis of photoreceptive disks, it is proposed that prominin has a role in the generation of plasma membrane protrusions, their lipid composition and organization and their membrane-to-membrane interactions.