A cancerous mole typically looks different from your other moles and shows one or more warning signs: an asymmetrical shape, irregular borders, uneven color, a diameter larger than 6 millimeters (about the size of a pencil eraser), or a noticeable change over weeks to months. These five features, known as the ABCDE rule, are the standard screening tool used by dermatologists and the National Cancer Institute to flag moles that need a closer look. No single feature guarantees cancer, but the more of these traits a mole has, the more suspicious it becomes.
The ABCDE Rule, Explained
Each letter corresponds to a specific visual feature of melanoma, the most dangerous form of skin cancer.
- Asymmetry. If you drew a line down the middle of the mole, the two halves wouldn’t match. Normal moles are roughly symmetrical.
- Border. The edges look ragged, notched, or blurred rather than smooth and well-defined. The pigment may seem to bleed into the surrounding skin.
- Color. Instead of one uniform shade of brown, you see a mix of brown, tan, black, or even patches of white, gray, red, pink, or blue within the same mole.
- Diameter. The mole is wider than 6 millimeters, roughly the size of a pencil eraser. Melanomas can be smaller than this, but larger lesions carry a notably higher risk. One study found that biopsied lesions over 6 mm had a positive predictive value of 11.5%, compared to just 2.6% for smaller ones.
- Evolving. Any change in size, shape, color, or height over weeks or months is a red flag. New symptoms like itching, bleeding, or scabbing also count.
Of these five, evolution is often the most reliable signal. A mole that looked the same for ten years and suddenly starts changing deserves prompt attention, even if it doesn’t check every other box.
The “Ugly Duckling” Approach
Most of your moles probably look similar to one another in size, shape, and color. The ugly duckling sign is a simple concept: if one mole stands out as clearly different from the rest, it warrants suspicion. You don’t need to memorize every detail of the ABCDE rule to use this method. Just scan your skin and look for the outlier, the one that doesn’t belong in the group. Dermatologists use this approach alongside the ABCDE criteria because it catches melanomas that might not fit neatly into a single checklist item.
Types That Don’t Follow the Rules
Nodular Melanoma
Not all melanomas spread outward across the skin first. Nodular melanoma grows downward into the skin rapidly, which means it can look like a small raised bump rather than a flat, oddly shaped mole. It often lacks the classic ABCDE features. Instead, look for the EFG pattern: elevated above the skin surface, firm to the touch, and growing noticeably over days to weeks. These tend to be more aggressive, so a new firm bump that’s getting bigger quickly should not be dismissed.
Melanoma Without Pigment
Some melanomas produce little or no pigment at all. These amelanotic melanomas often look like a pink or red bump, patch, or nodule on the skin. Because they don’t match the typical dark, multicolored appearance people expect, they’re easy to mistake for a pimple, scar, or irritation. If a pink or reddish spot persists for weeks, grows, or doesn’t heal, it’s worth having it examined.
Melanoma on Palms, Soles, and Nails
Acral lentiginous melanoma appears on the palms of the hands, soles of the feet, or under fingernails and toenails. It accounts for the majority of melanoma diagnoses in people of color, and it occurs equally across all racial backgrounds. On the palm or sole, it starts as a black or brown discoloration that may resemble a bruise or stain but grows over time. Under a nail, it typically appears as a dark vertical streak running the length of the nail bed. As it progresses, it can cause the nail to crack or break. This type is frequently mistaken for a fungal infection or a bruise from injury, which delays diagnosis.
Who Faces Higher Risk
Your baseline risk goes up significantly if you have atypical moles, sometimes called dysplastic nevi. These are moles that are larger than average, have irregular borders, or show uneven coloring but aren’t cancerous themselves. Having more than five of these atypical moles makes your melanoma risk roughly ten times greater than someone with none. People who have a large total number of moles also face increased risk, even if each individual mole looks relatively normal.
Other factors that raise your risk include a history of severe sunburns (especially in childhood), a family history of melanoma, fair skin that burns easily, and a weakened immune system. But melanoma can develop in anyone regardless of skin tone, which is why checking areas like the soles of your feet and nail beds matters for everyone.
What a Skin Check Looks Like
A thorough self-exam means checking your entire body, not just sun-exposed areas. Stand in front of a full-length mirror and use a hand mirror for your back, scalp, and the backs of your legs. Check between your toes, the soles of your feet, your palms, your nail beds, and behind your ears. You’re looking for any mole that meets the ABCDE criteria, any outlier that looks different from the rest, and any new spot that appeared recently or changed.
When a dermatologist examines your skin, they use a handheld magnifying device called a dermatoscope. This tool dramatically improves accuracy. Visual inspection alone detects about 76% of melanomas, but adding dermoscopy raises detection to 92% while also reducing false alarms. The specificity jumps from 75% to 95%, meaning far fewer harmless moles get flagged unnecessarily. This is a major reason why professional skin exams catch things self-checks miss.
What Happens if a Mole Looks Suspicious
No one can diagnose melanoma just by looking at it, not even a dermatologist. The only definitive answer comes from a biopsy, where a sample of the mole is removed and examined under a microscope. There are a few ways this is done.
An excisional biopsy removes the entire mole along with a small margin of healthy skin around it, using a scalpel. This is generally the preferred approach when melanoma is suspected because it gives the pathologist the full picture. It usually requires stitches. A punch biopsy uses a small circular cutting tool to remove a deeper core of skin. It’s sometimes used when the mole is in a location where full excision is difficult. Stitches may or may not be needed depending on the size.
The procedure itself is done under local anesthesia, so you’ll feel pressure but not pain. Results typically come back within one to two weeks. If melanoma is confirmed, the pathology report will include details about how deep the cancer has grown, which determines the next steps for treatment.
Most biopsied moles turn out to be benign. But since early-stage melanoma is highly treatable and late-stage melanoma is far more dangerous, the threshold for doing a biopsy is intentionally low. A mole that turns out to be nothing is a much better outcome than a melanoma that gets ignored.