Protease responsible for filaggrin processing, essential for the maintenance of a proper epidermis organization. Homodimer. Highly expressed in stratified epithelia in skin, tongue, esophagus, forestomach and vagina. Also expressed in trachea, urinary bladder and thymus. Undetectable in simple epithelia. Within the epidermis, expressed exclusively in the granular layer (at protein level). Levels are elevated in benign skin tumors but are down-regulated in squamous cell carcinomas. Expression is first detected at embryonic day 15.5. By 12-O-tetradecanoylphorbol-13-acetate (TPA). No TPA-induced expression is seen in mice lacking Fos. Undergoes autocleavage which is necessary for activation of the protein. Mice display fine wrinkles on the lateral trunk which start to form 5 weeks after birth. There are no apparent epidermal differentiation defects.