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Ichthyosis: Dry, rectangular scales on the skin. Like fish scales. From the Greek "ichthys" meaning fish.

Ichthyosis is not one but rather a group of skin disorders characterized by noninflammatory scaling of the skin. Many different metaphors have been used to describe the appearance and texture of the skin in the various types and stages of ichthyosis, for example alligator skin, crocodile skin, collodion skin, porcupine skin and, of course, fish skin.

Most ichthyoses are genetic (inherited). Some ichthyoses develop in association with various systemic diseases or be a prominent feature in certain genetic syndromes. The term "ichthyosis" is commonly used alone to refer to ichthyosis vulgaris.

Ichthyosis vulgaris, also known as ichthyosis simplex, is inherited as an autosomal (nonsex-linked) trait and is characterized by scaly (fishlike) areas of skin. The first scaly skin problems usually appear after 3 months of age. The palms and soles are often affected. Areas that tend to be spared include the axillae (the armpits), the antecubital fossa (the inside area at the bend of the elbow) and the popliteal fossa (behind the knee).

The gene responsible for ichthyosis vulgaris has been mapped to chromosome band 1q21. The product of this gene is thought to be a substance called filaggrin (abbreviated FLG) which may act as the "keratin matrix protein" in cells of the stratum corneum, one of the layers of the skin.

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