Keratin is found in the body in two basic types: hard and soft keratin. Hard keratin is found in hair and contains high levels of cysteine, while soft keratin (epidermal keratin) contains substantially lower levels of cysteine. As such, epidermal keratin is less stable to alkaline conditions, reducing substances, heat, and proteolytic enzymes than hard keratin. Epidermal keratin comprises more than half the dry weight of human cornified epithelium. The protein is insoluble in water and physiologic buffer solutions. Studies show that keratins are useful in the identification of certain types of tumors.