Skin Cancer How To Tell If You Have It The ABCDE rule is a mnemonic used to help identify potential signs of skin cancer. Each letter represents a key characteristic to look for when examining a mole or other skin lesion: A - Asymmetry: One half of the lesion looks different from the other half. B - Border: The edges of the lesion are irregular, ragged, or blurred. C - Color: The lesion has different colors or shades, such as black, brown, tan, white, red, or blue. D - Diameter: The lesion is la