A Congenital Melanocytic Naevus CMN (spelt Nevus or Naevus) is a skin growth present at birth. It is a rare disorder that affects around 1 in 20,000 newborns in some form and as few as 1 in 500,000 in its most severe form. It is composed of special pigment producing cells called melanocytes, which are the pigment producing cells normally found in skin but which behave differently in CMN. They can exist on any part of the body, and grow in proportion to body growth. There are several definitions used to refer to the size of a congenital nevus, from small, medium, large or giant. A giant nevus is one which covers a large portion of an anatomical site, for example scalp, face, arm, leg or back. Another definition is if it covers more than 2% of a patients total body area or will measure over about twenty centimetres when the patient is fully grown. Many large or giant nevi are far more extensive than this. Many patients also have multiple other nevi called satellite nevi on other parts of their body, and some will continue to develop these over their lifespan. Most Nevi are brown or black in colour, and have an increased tendency for hair growth. The skin texture can be smooth or leathery and because oil and sweat glands do not form properly within the nevus, overheating can be a problem. The skin is often dry and fragile. Many nevi have tumours just below the skin that give the nevus a lumpy or folded appearance.