This is a type of skin cancer. The cancer is due to abnormal growth and spread of melanocytes in the skin. Melanocytes are a type of skin cell and are responsible for producing brown pigment when exposed to the sun. It is this brown pigment which gives us a 'sun tan'. Groups of melanocytes often form coloured lumps, called moles , in the skin.
Ordinary moles are quite harmless.
However, occasionally, the cells in a mole can become cancerous and start to grow and spread. This cancer is called a malignant melanoma. As the cancer grows, some of the cancer cells can spread to other areas of the body. If not treated early, the liver, lungs, brain, abdomen, and other parts of the body, can become invaded by cancer cells. The main cause of malignant melanomas is exposure of your skin to strong sunlight.