Epidermolytic nevus (EN) is visible as patches of thickened skin over small areas of the body. Mutations in genes encoding the skin proteins keratin 1 or keratin 10 are responsible for EN, but these mutations only occur in some cell populations of the body, including those of skin lesions, so they are known as mosaic. When the causative mutation occurs in germ cells, it is known as gonadal mosaicism. Birthmarks are not usually inherited because sperm cell genes are rarely mutated. However, when inheritance does occur, the children develop skin symptoms identical to their affected parent but covering their entire body.