Epithelial tissue covers a person's inner and outer surfaces, forms secretory glands and acts as a barrier between the organism and the external environment. Changes to the genetic makeup of proteins such as keratin, collagen and laminin, which provide elasticity and integrity, can lead to breakage and fractures. As epithelial tissue is found throughout the body, many organs can be affected simultaneously, particularly in cases of hereditary transmission. This section discusses some of the most common rare hereditary epithelial tissue diseases. However, it should be noted that there are other such diseases besides those discussed here. One of the best-known and most severe groups of epithelial tissue diseases is Epidermolysis Bullosa (EB), commonly known as 'Butterfly Disease'. EB is a group of rare, genetically inherited conditions characterised by the fragility of the affected tissues, and there is currently no cure. The skin and mucous membranes are so fragile that even slight friction or touch can lead to severe blistering and open wounds. Ichthyosis, also known as fish scale disease, is characterised by a disruption to the normal shedding process of epithelial cells and excessive accumulation of keratin. The skin becomes excessively dry and hard with a fish-scale appearance. Darier's disease (keratosis follicularis) is caused by a mutation in the ATP2A2 gene, which codes for calcium pumps. This damages the 'desmosome' structures that allow cells to stick together. The condition manifests as hard, foul-smelling bumps that resemble warts, especially on the chest, back and forehead. Pachyonychia congenita is a rare genetic condition that affects keratin production. The most prominent feature is the excessive thickening of the nails. It also causes painful, callus-like keratoses to develop on the palms and soles of the feet, which can make walking difficult or even impossible. Ectodermal dysplasia is a group of diseases that affect the skin epithelium and the development of ectodermally-derived structures, such as hair, nails, teeth and sweat glands. The absence of sweat glands poses a major life-threatening risk of hyperthermia in these individuals.