Moles are common skin lesions. They are correctly called melanocytic naevi (American spelling ‘nevi') as they are due to a proliferation of the pigment cells, melanocytes. If they are brown or black in colour, they may also be called pigmented naevi. Moles are benign in nature (harmless), but a malignant melanoma (cancerous mole) may arise within a mole.
Naevi may form from other skin cells (e.g. vascular naevi are formed from blood vessels), but only those derived from melanocytes are known as moles.
Moles may be flat or protruding. They vary in colour from pink or flesh tones to dark brown or black. Although mostly round or oval in shape, they are sometimes unusual shapes. They range in size from a couple of millimetres to several centimetres in diameter.
The number of moles a person has depends on genetic factors and on sun exposure; most white-skinned New Zealanders have 20-50 of them. People with a greater number of moles have a higher risk of developing melanoma than those with few moles, especially if they have over 100 of them.
One or more moles may be present at birth. These brown birthmarks are more correctly known as congenital melanocytic naevi. If birthmark-like moles appear within the first two years of life, they are sometimes called 'congenital-type' melanocytic naevi.
More frequently moles arise during childhood or early adult life, when they are called acquired melanocytic naevi. Exposure to sunlight increases the number of moles. Teenagers and young adults tend to have the greatest number of moles and there are fewer in later life because some of them slowly fade away.
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