types I and II are the keratins which are found in various combinations in epithelial cells throughout the body. Keratin IFs must include representative subunits of both type I and type II, with each tissue having a characteristic combination. In contrast, type III IF subunits typically form homopolymers. They include IFs that are characteristic of less differentiated cells like glial or neuronal precursors, as well as those seen in more specialized cell types like smooth muscle cells and mature astrocytes. These three types of IF not only share sequence homology within their rod domains, but their genes exhibit similar exon and intron structure. All type IV IFs are neuron-specific and have a common pattern of exons and introns that differs from that seen in types I—III. Finally, type V polypeptides are the nuclear lamins which form the walls of the nuclear envelope structure, rather than typical IF structures. While all eukaryotes express type V lamins, the other IF types are not found in plants, many unicellular organisms and the arthropods. Cytoplasmic IFs are, however, nearly ubiquitous in vertebrate cells, and some cells contain more than one type of IF in the cytoplasm. Curiously, oligodendrocyte precursors contain vimentin IFs, but these are lost during differentiation, making mature oligodendrocytes one of the few vertebrate cell types that lack cytoplasmic IFs.