Keratin 7 is a type II keratin. Filaggrin binds to keratin fibers in epidermal cells. The protein keratin stiffens epidermal tissue to form fingernails. Alternatively, these proteins can interact with keratin intermediate filaments. Concentrated nitric acid stains human skin yellow due to its reaction with the keratin. Again, the cysteine is used for breaking up the disulfide bonds in the hair's keratin. It is an important biomaterial primarily composed of protein, notably keratin. Formaldehyde is a common ingredient in Brazilian blowouts, Cadiveu, and Keratin Complete Smoothing Therapies. Desmosomes attach to the microfilaments of cytoskeleton made up of keratin protein. The cytoplasm is released and the protein keratin is inserted. Baleen whales have bristles made of keratin instead of teeth. A true claw is made of a hard protein called keratin. Hedgehogs are easily recognized by their spines, which are hollow hairs made stiff with keratin. Adapalene has the unique ability to inhibit keratinocyte differentiation and decrease keratin deposition. KRT37 is the only keratin that is regulated by androgens.