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It is a rare form of skin cancer. It usually starts on the glans (tip) of the penis, or on the foreskin (the skin that covers the end of the penis).
Cancer occurs when normal body cells get out of control, multiply and spread. These cells clump together to form a cancerous or malignant tumour which can then grow and damage healthy parts of the body.
For a photograph of the penis, with the glans and foreskin labelled please visit: http://upload.wikimedia.org/wikipedia/en/1/1d/Labelled_bw_flaccid_penis.jpg
