A large proportion of the cytoplasm of vertebrate cells, normal or transformed, is represented by components of the cytoskeleton, including actin-containing microfilaments, tubulin-containing microtubules and filaments of intermediate size, with diameters of 7-l 1 nm. Although such structures have a widespread occurrence in diverse cell types, examples have been reported in which they are formed in different cell types from different proteins of a multigene family of proteins, or from different subunit polypeptides of a class of related proteins. For example, differentiation specificity of expression of different actins has been described in different cell types of mammals (Vandekerckhove and Weber, 1979). By far the most striking differentiation specificity of composition has been observed for the intermediate-sized filaments. Although all filaments of this category are morphologically identical in different cell types, are insoluble in solutions of a broad range of low or high salt concentrations and non-ionic detergents and seem to share some common assembly properties (Steinert et al., 1981 b) and antigenic determinants (Pruss et al., 1981) immunological and biochemical criteria allow us to distinguish at least five different types of intermediate filaments (Bennett et al., 1978; Franke et al., 1978a, 1981f; Hynes and Destree, 1978; Lazarides, 1980; Anderton, 1981; Holtzer et al., 1981; Osborn et al., 1981). First, filaments containing keratin-like proteins (“cytokeratins”) are characteristic of epithelial cells. Second, vimentin filaments occur in mesenchymally derived cells, in astrocytes, in Sertoli cells, in vascular smooth muscle cells and in many cultured cell lines. Third, desmin filaments are typical of most types of myogenic cells. Fourth, neurofilaments are typical of neuronal cells. Fifth, glial filaments are typical of astrocytes. During cell transformation and tumor development this cell type specificity of intermediate filaments is largely conserved’(Franke et al., 1978a, 1978b, 1979a; Hynes and Destree, 1978; Sun and Green, 1978a; Sun et al., 1979; Bannasch et al., 1980; Battifora et al., 1980; Schlegel et al., 1980a; Altmannsberger et al., 1981; Gabbiani et al., 1981; Denk et al., 1982) and classification of tumors by their specific type of intermediate filaments has recently become very valuable in clinical histodiagnosis (see, for example, Schlegel et al., 1980a; Gabbiani et al., 1981; Ramaekers et al., 1981). The intermediate filaments of the vimentin, desmin or glial types all consist usually of only one type of subunit protein (desmin and vimentin can occur in the same filament in BHK cells and vascular smooth muscle cells; Steinert et al., 1981 a; Quinlan and Franke, 1982). In contrast with these, the cytokeratin filaments, which are composed of proteins related to, but not identical with, epidermal (Y keratins, are a complex family of many different polypeptides. These cytokeratins, which show biochemical and immunological relationships of various degrees, are expressed, in different epithelia, in different combinations of polypeptides ranging in their isoelectric pH values from 5 to 8 and in their apparent molecular weights from 40,000 to 68,000 (Doran et al., 1980; Winter et al., 1980; Fuchs and Green, 1980, 1981; Franke et al., 1981 a, 1981 b, 1981 c; Milstone and McGuire, 1981; Wu and Rheinwald, 1981). A given epithelium or epithelial cell can therefore be characterized by the specific pattern of its cytokeratin components.