The more numerous cells are called keratinocytes which produce a tough, fibrous, waterproof protein called keratin. Keratinization is the process in which cells form fibrils of keratin and harden. Over most of the body, keratinization is minimal, but the palms of the hands and the soles of the feet normally have a thick, outer layer of dead, keratinized cells. Melanocytes, which are less numerous than keratinocytes in the epidermis, produce a pigment called melanin. Melanin ranges in color from yellow to brown to black and determines skin color.