The primary protein found in hair and nails is keratin. Keratin is a fibrous structural protein that provides strength and resilience to these structures. It is also a crucial component of skin, feathers, horns, and other hard or protective tissues in various animals. Hair and nails are composed of hard keratin, which differs from the softer keratin found in other parts of the body. The high content of sulfur in keratin contributes to its toughness and provides protection against wear and damage. Keratinization is the process by which cells produce keratin, leading to the formation of these tough structures.