Since the 1930s, the properties and structures of the filamentous component of wool and related keratins have been the subject of various studies (Astbury & Street, 1931). However, only within the last few years has it become apparent that these proteins are members of a larger class of fibrous proteins, related by their common morphologies, that form a component of the cytoskeleton of virtually all vertebrate cells. These are now known as intermediate filaments (IF), and consist of a heterogeneous population of protein subunits. Based on the patterns of tissue distribution, and on immunological and biochemical properties, the current trend is to use a classification system for IF that defines five major subclasses of subunits that can form IF in vitro and in vivo (Lazarides 1980, 1982; Zackroff et al 1981; Steinert 1981; Steinert et a11984a, 1985a; Yang et aI1985). These subclasses include one, a large family of keratin subunits of about 40-70 kilodaltons (kDa) two or more members of which are coexpressed in most epithelial cells. A second subclass is found in myogenic cells and consists of a single protein of about 52 kDa called desmin (or skeletin). A third subclass consists of the 50 kDa glial fibrillary acidic protein, which is found in astroglial tissues. Many cells of mesenchymal origin, or cell lines established in culture, contain a protein, vimentin, of about 53 kDa, which defines the fourth subclass. The fifth subclass is found in neuronal tissues and consists of varying proportions of three" triplet" subunits of about 60-70 kDa (the NF-L subunit), 105-110 kDa (NF-M), and 135-150 kDa (NF-H). In addition, some cells can also express vimentin in addition to their more specific IF subclass.

There is growing evidence that invertebrate cells also contain IF or IF like proteins, eg as neurofilaments in the squid brain, or in the giant axons of marine worms (Zackroff et al 1981); in Drosophila cells (Walter & Biessmann 1984); and in Dictyostelium discoideum (Koury & Eckert 1984). Thus, IF may be ubiquitous constituents of the cytoskeletons of all eukaryotic cells. Further, there is a growing list of proteins associated with IF (IF AP), or that share certain properties with the subclasses enumerated above (Lazarides 1982; Wiche et al 1982; Zackroff et al 1984; Yang et al 1985). Accordingly, as more is learned about IF, it may eventually be necessary to adopt a more systematic nomenclature system for all of these