Keratin dimers associate in antiparallel orientation to form tetramers (Wood & Inglis 1984, Quinlan et al. 1984). The rod regions of heterodimers align, but alignment of the head and tail regions differs between keratin types. Soft-keratinizing-cornifying cell keratins are slightly out of phase, by 7-8 amino acids, while keratin heterodimers of hard-keratinizing-cornifying cells are in register and consequently there is no overlap between the head and tail domains when a tetramer is formed (Jones et al. 1997). Protofilament tetramers have a diameter of about 2 nm (Aebi et al. 1983, Eicher & Kahn 1990). Heterodimers and tetramers represent the stable building blocks of larger octamers (Herrmann & Abei 2004) and Unit Length Filaments (ULFs), which have a diameter of 20 nm (Parry et al. 2001, Hermann et al. 2007). The tetramers are stabilized by a hydrophobic stripe exposed at the surface of coiled-coil keratin heterodimers (Bernot et al. 2005).