Protease responsible for filaggrin processing, essential for the maintenance of a proper epidermis organization. Homodimer. Expressed primarily in the granular layer of the epidermis and inner root sheath of hair follicles. In psoriatic skin, expressed throughout the stratum corneum. In ulcerated skin, expressed in the stratum granulosum of intact epidermis but almost absent from ulcerated regions. Expressed in differentiated areas of squamous cell carcinomas but not in undifferentiated tumors. Undergoes autocleavage which is necessary for activation of the protein. The disease is caused by variants affecting the gene represented in this entry. It is uncertain whether Met-1 or Met-85 is the initiator.