Melanoma
Less common than other types, melanoma is by far the most dangerous, causing about 75 percent of all skin cancer-related deaths (American Melanoma Foundation, 2009). It occurs in the skin cells that create pigment, and it creates moles or lesions that follow an ABCDE pattern in their irregularities:
asymmetrical shape
border irregularities
color
diameter
evolution of the lesion
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http://en.wikipedia.org/wiki/Melanoma