Melanocytes are found under the skin, which is made up of two layers: the epidermis on the outside and the dermis below that. To be accurate, melanocytes are found in the lowest levels of the epidermis, but not actually in the dermis. These cells produce melanin, which affects the epidermis' pigmentation, both natural skin colour and because of exposure to the sun as in tanning.

Sometimes, a group of near-by melanocytes combine with a little local tissue to form a mole (also known as a nevus; plural nevi). The average person has between ten and forty moles, which usually appear before the fortieth birthday. They often fade or disappear with age. Moles are non-malignant (non-cancerous) and can be flat or raised in shape and almost any colour. Usually, they are slightly darker than one's natural skin colour. Dark skinned people tend to have more moles.

Cancer begins in cells where the normal cycle of decay and replacement by regeneration has been disrupted. In these circumstances, cells do not always die when they should and new cells are produced needlessly. This, in turn, produces a growth (also known as a tumor), which can be either benign or malignant (ie cancerous or non-cancerous).

Benign tumors can be surgically removed and rarely return. They do not spread or affect surrounding tissue. Malignant tumors are cancerous and can affect surrounding tissue and organs. In these cases, cancerous cells can break away from the primary tumor and affect other organs or enter the blood stream (lymphatic system), whereby it will spread to their parts of the body (metastasis) very quickly. The rate of metastasis is a deciding factor in how a doctor treats cancer.

Melanoma occurs when melanocytes become malignant. It can occur at any age, but chances increase with age. Fair-skinned people are more likely to develop it than dark-skinned people. In fair-skinned races, men tend to get it on the torso and neck, whereas women get it on their calves (lower legs). Dark-skinned people rarely get melanoma, but if they do, it is usually under the finger and toe nails or on the soles of the feet or palms of the hands. When cancerous cells from melanoma enter the lymphatic system and affect other organs, it is still attributed to melanoma. For example, if the liver becomes affected by cancerous cells from melanoma, it is referred to as metastatic melanoma, not liver cancer.

Often, the first sign of melanoma is a change in the size, shape, colour, or feel of an existing mole, although it often first manifests itself with a new mole or moles. Self-diagnosis is not to be relied on - always seek professional advice if you have any concerns relating to your skin. However, it is wise to remember 'The ABCD of Melanoma', which goes thus:

Asymmetry: the shape of one half of the mole is not the same as the other half.

Border: the border or edges of the mole are not clearly defined; a bit ragged or the pigmentation 'leaks' into the surrounding skin.

Colouration: the mole is not uniformly of one colour, although it is not so important what that colour is.

Diameter: there is a change in size or a new mole grows larger than 5mm in size.