DR P. MARAZZI / SCIENCE PHOTO LIBRARY DR P. MARAZZI / SCIENCE PHOTO LIBRARY
Large birthmark, or pigmented naevus, on the arm of a 15-year-old boy. A pigmented naevus is caused by abnormality or overactivity of melanocytes, the skin cells that produce the brown pigment melanin. Most naevi are harmless (benign) and do not require treatment. However, in some cases a naevus may turn cancerous and will require surgical removal.
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