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The most important warning sign for melanoma is a change in size, shape, or color of a mole or other skin growth, such as a birthmark. One of the things to watch for is diameter.

Diameter is width. If the diameter across a mole or skin growth is larger than 1/4 inch (6 mm), or about the size of a pencil eraser, talk to your doctor. Also, call your doctor if any mole seems to be growing, even if it is still smaller than 1/4 inch (6 mm).