Structure of keratin. Illustration of the structure of the protein keratin, as found in strands of hair. At top, the smallest unit shown here is the alpha helix of the protein. These combine to form a protofibril (upper centre), which in turn combine to form a microfibril (lower centre). These form macrofibrils which are shown in a hair cell (bottom) within a strand of hair. Keratin is also found in skin and nails. For this artwork with labels, see image C023/8711.