Mammalian keratins form soluble short full-width filaments called unit length full-width particles (Parry et al. 2007), unit length filaments (ULFs) (Herrmann et al. 2002) or intermediate filament-like particles (Steinert 1991). These are formed by the lateral association of tetramers. ULFs are ~ 70 nm long, with a diameter of ~20 nm. The diameter shrinks during formation of filaments (Parry et al. 2001). X-ray diffraction suggests that ULFs are tube-like structures formed from eight tetramers in non-cornified cells (Parry et al. 2007). In cornified hair cells, the tetramers are thought to be arranged in a seven-member ring, with an eighth in the centre (Parry et al. 2007).