A bad mole typically looks different from the rest of your moles. It may be asymmetrical, have jagged or blurry edges, contain multiple colors, measure larger than 6 millimeters (about the size of a pencil eraser), or change noticeably over weeks to months. These five features form the ABCDE rule, which is the most widely used framework for spotting a potentially cancerous mole. But not every dangerous mole follows these rules, and some warning signs are easy to miss if you don’t know where to look.
The ABCDE Rule
Each letter stands for a visual feature that distinguishes a normal mole from one that could be melanoma.
- Asymmetry. If you drew a line through the middle, one half wouldn’t match the other. Normal moles are generally round or oval and roughly symmetrical.
- Border. The edges are ragged, notched, or blurred rather than smooth. The pigment may visibly spread into the surrounding skin.
- Color. Instead of one uniform shade of brown, you see a mix of colors: black, brown, tan, and sometimes areas of white, gray, red, pink, or blue.
- Diameter. Most melanomas are larger than 6 millimeters when diagnosed, roughly the width of a pencil eraser. That said, melanomas can be smaller than this, so size alone doesn’t rule anything out.
- Evolving. The mole has changed in size, shape, or color over the past few weeks or months. Any mole that looks different than it did recently deserves attention.
A mole doesn’t need all five features to be suspicious. Even one, especially evolving, is enough reason to get it checked.
The Ugly Duckling Sign
Most people’s moles share a general “family resemblance.” They tend to be similar in size, shape, and color. The ugly duckling sign refers to one mole that looks obviously different from all the others on your body. It stands out the way one odd bird would in a group of identical ducklings. This is a useful shortcut because it doesn’t require you to memorize clinical criteria. If a mole catches your eye because it simply doesn’t match the rest, that instinct is worth following up on.
Physical Symptoms Beyond Appearance
A dangerous mole doesn’t always announce itself through looks alone. New itching, pain, or tenderness in a mole can signal a problem. Bleeding or oozing without any injury is particularly concerning, as is a mole that develops a crust or scab that won’t heal. These surface-level changes sometimes appear before the visual signs become obvious, which makes them valuable early warnings.
Moles That Don’t Look Like Moles
Not every melanoma is a dark, irregularly shaped spot. About 2% of melanomas are amelanotic, meaning they have little or no pigment. These often appear as a pink or red bump, a flesh-toned papule, or a small raised nodule. Because they don’t look like the typical “bad mole” people picture, they’re frequently mistaken for pimples, bug bites, or harmless skin growths. If you have a pink or reddish bump that doesn’t heal within a few weeks or keeps coming back, it’s worth having a dermatologist look at it.
Nodular melanoma is another type that breaks the usual pattern. Rather than spreading outward across the skin’s surface, it grows downward, appearing as a firm, dome-shaped bump. It develops quickly, often over just several weeks, and can be dark brown, black, blue-black, or even skin-colored. Because it grows fast and deep, nodular melanoma is one of the more aggressive forms, making early recognition especially important.
Hidden Locations Most People Miss
Melanoma can develop in places you’d never think to check. On the palms of your hands or soles of your feet, it appears as a black or brown discoloration that may initially resemble a bruise or stain. Over time it grows in size and becomes more irregular. This type, called acral lentiginous melanoma, is the most common form of melanoma in people with darker skin tones.
Under the fingernails or toenails, melanoma usually shows up as a dark vertical streak running from the base of the nail toward the tip. The streak is typically blackish brown with irregular coloring and may widen over time, especially at the nail’s base. One key warning sign is discoloration that spreads from the nail onto the surrounding skin, known as the Hutchinson sign. In some cases, there’s no visible streak at all. Instead, a small growth develops under the nail, lifting or damaging it. These changes are often mistaken for a fungal infection or dried blood from an injury, which can delay diagnosis.
Atypical Moles vs. Melanoma
Some people have moles that look unusual but are still benign. These are called dysplastic nevi, or atypical moles. They can be flat with a slightly pebbly surface, irregularly shaped, larger than a pencil eraser, and contain a mix of pink, red, tan, brown, and black. On paper, that description sounds a lot like melanoma, and the overlap can be confusing.
The key distinction is stability. An atypical mole may look odd, but it stays the same over time. A melanoma evolves. It changes shape, grows wider, shifts color, or develops new symptoms like bleeding or itching. If you have several atypical moles, you’re at higher risk for melanoma, which makes regular monitoring even more important. Knowing what your moles normally look like is the baseline that lets you spot when something changes.
How to Check Your Skin
The Skin Cancer Foundation recommends a head-to-toe self-exam once a month. This means checking not just your arms and legs but also your scalp (use a blow dryer to part your hair), the spaces between your toes, the soles of your feet, your nail beds, and your back using a hand mirror or a partner’s help. The goal isn’t to diagnose anything yourself. It’s to build a mental map of your moles so you notice when something is new or different.
A professional full-body skin exam by a dermatologist once a year fills in the gaps. They can evaluate moles in hard-to-see areas and use magnification tools to examine spots that look ambiguous. If you have a personal or family history of melanoma, or if you have many atypical moles, your dermatologist may recommend more frequent visits.