Chapter 1 25 in the KIF proteins of the heptadic repeat of amino acids with hydrophobic ones at positions a and d of the heptads. With such a coiled-coil arrangement of the dimer, the hydrophobic side- chains of the amino acids face each other at the interface between the two α-helices. Attraction between charges of opposite sign for amino acid side chains located at positions e and g of the heptads not only holds the coiled-coil structure in place but serves also to ensure a parallel in-register alignment of the two α-helices. This is shown schematically in Figure 13 which is a view looking down the axis of the coiled-coil. Little doubt remains that this type-I/ type-II α-helical coiled-coil dimeric rod of approximately 70 Å in length, with globular cystine rich ends, is the basic building block from which the complete intermediate filament of the hair keratin composite is constructed (Figure 14). The scanning transmission electron microscope has been used to measure the mass per unit length of keratin IFs isolated from human head hair follicles. By this means it was established there was a strong possibility of there being 32 protein chains (16 coiled-coil dimers) in the filament’s transverse section.60, 61 Figure 12. The amino acid sequence for a type I KIF protein from human hair.