The reticulon family is a large and diverse group of membrane-associated proteins found throughout the eukaryotic kingdom. All of its members contain a carboxy-terminal reticulon homology domain that consists of two hydrophobic regions flanking a hydrophilic loop of 60-70 amino acids, but reticulon amino-terminal domains display little or no similarity to each other. Reticulons principally localize to the endoplasmic reticulum, and there is evidence that they influence endoplasmic reticulum-Golgi trafficking, vesicle formation and membrane morphogenesis. However, mammalian reticulons have also been found on the cell surface and mammalian reticulon 4 expressed on the surface of oligodendrocytes is an inhibitor of axon growth both in culture and in vivo. There is also growing evidence that reticulons may be important in neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis. The diversity of structure, topology, localization and expression patterns of reticulons is reflected in their multiple, diverse functions in the cell.