Hair and the outer skin and nails are all composed largely of the same protein, which is:

Hair, the outermost layer of skin, and the nails are all composed largely of keratin, a tough, fibrous, insoluble protein. Keratin gives these structures their strength and protective qualities. This shared composition is why hair and nails are both classified as keratinized structures. (Collagen is a structural protein of the dermis; hemoglobin and insulin are unrelated body proteins.) Recognizing keratin as the common protein of hair, nails, and the skin's surface links several scientific-concepts topics tested on the exam.