The most reliable way to check a mole for signs of cancer is the ABCDE rule: look for asymmetry, irregular borders, uneven color, a diameter larger than 6mm (about the size of a pencil eraser), and any evolution or change over time. A mole that hits one or more of these markers doesn’t necessarily mean cancer, but it does mean you should have a dermatologist look at it. Early detection matters enormously. Melanoma caught before it spreads has a five-year survival rate of about 98%, compared to roughly 16% once it reaches distant parts of the body.
The ABCDE Rule, Explained
This five-letter system was developed in the 1980s and updated in 2004. It gives you a structured way to evaluate any mole on your body.
- Asymmetry: If you drew a line down the middle of the mole, one half wouldn’t match the other. Normal moles tend to be roughly symmetrical.
- Border irregularity: The edges are ragged, notched, or blurred rather than smooth and well-defined. The pigment may seem to bleed into the surrounding skin.
- Color variation: Instead of a single uniform shade of brown, the mole contains a mix of colors. Shades of black, brown, and tan are common, but patches of white, gray, red, pink, or blue are especially concerning.
- Diameter: Most melanomas are larger than 6mm across, about the width of a pencil eraser. However, this is not a hard rule. Research shows that roughly 22% of melanomas are 6mm or smaller, and nearly 20% of those small melanomas have already grown deep enough into the skin to be dangerous. Some experts have proposed replacing the “diameter” criterion with “dark,” since any unusually dark lesion deserves attention regardless of size.
- Evolving: The mole has changed in size, shape, color, or texture over the past few weeks or months. New symptoms like bleeding, itching, or crusting also count as evolution.
No single letter on its own confirms cancer. A mole can be slightly asymmetrical and perfectly harmless. But the more criteria a mole meets, and the more dramatic those features are, the higher the concern.
The Ugly Duckling Sign
Beyond ABCDE, there’s a simpler and surprisingly useful concept. Most of your moles probably look similar to each other in color, size, and shape. The “ugly duckling sign” is exactly what it sounds like: one mole among many that just looks different from the rest. It stands out. That outlier deserves a closer look, even if it doesn’t clearly match the ABCDE criteria. Sometimes melanoma doesn’t follow the textbook pattern, and this gut-level “something’s off” comparison catches what the checklist might miss.
What About Moles That Aren’t Melanoma?
Not every suspicious-looking mole is melanoma. Atypical moles (also called dysplastic nevi) can share several features with melanoma: irregular shape, blurry edges, mixed colors, and a size larger than a pencil eraser. These moles are benign but can look alarming. The key difference is stability. An atypical mole that has looked the same for years is far less concerning than one that’s actively changing. That said, having many atypical moles does increase your overall melanoma risk, so it’s worth establishing a baseline with a dermatologist.
Skin cancer also comes in forms that don’t look like moles at all. Basal cell carcinoma, the most common type, often appears as a skin-colored or pink bump on lighter skin, or a brown or glossy black bump with a rolled border on darker skin. It can also show up as a sore that heals and then reopens. Squamous cell carcinoma starts in a different layer of skin cells and can look like a rough, scaly patch or a firm red bump. Neither of these typically resembles a traditional mole, but both are worth catching early.
Melanoma on Darker Skin
Melanoma is often discussed in the context of sun exposure and fair skin, but one subtype, acral lentiginous melanoma, occurs equally across all races and skin tones. It accounts for the majority of melanoma cases in people of color, and it shows up in places that are easy to overlook: the soles of the feet, the palms of the hands, and under the fingernails or toenails.
On the palms or soles, it starts as a black or brown discoloration that may look like a bruise or stain. Unlike a bruise, it doesn’t fade. It grows over time. Under the nails, it appears as dark vertical streaks running the length of the nail bed and is sometimes mistaken for a fungal infection or dried blood. As it progresses, it can cause the nail to crack or break. These locations are easy to miss during casual self-checks, which is one reason melanoma in people of color is more likely to be diagnosed at a later stage.
How to Do a Self-Exam
The American Academy of Dermatology recommends checking your skin regularly. A full self-exam takes about 10 minutes and works best with a full-length mirror, a hand mirror, and good lighting. Here’s a systematic approach:
- Front and back: Stand in front of a full-length mirror and scan your entire body, then turn around.
- Arms: Raise your arms and check the right and left sides of your torso. Then bend your elbows and examine your forearms, underarms, fingernails, and palms.
- Legs and feet: Check the backs of your legs, the tops and soles of your feet, between your toes, and your toenails.
- Scalp and neck: Use a hand mirror to examine the back of your neck and scalp. Part your hair in sections to see the skin underneath.
- Back and buttocks: Use the hand mirror again (or ask a partner) to check areas you can’t easily see.
The goal isn’t to diagnose anything yourself. It’s to notice change. Taking photos of moles you want to track can make it much easier to spot subtle shifts in size, shape, or color over months.
What Happens if a Mole Looks Suspicious
A dermatologist will first examine the mole visually, often using a dermatoscope, which is essentially a magnifying lens with a light that reveals structures beneath the skin’s surface. If the mole looks concerning, the next step is a biopsy, meaning a small sample (or the entire mole) is removed and examined under a microscope. This is the only way to confirm whether a mole is cancerous.
The two most common biopsy techniques are the punch biopsy and the excisional biopsy. A punch biopsy uses a small circular blade to remove a core of skin, and may or may not need stitches depending on the size. An excisional biopsy uses a scalpel to cut out the entire mole along with a margin of healthy tissue around it, and typically does require stitches. When melanoma is suspected, dermatologists generally prefer to remove the whole mole rather than just a piece of it.
The biopsy itself is done under local anesthesia and usually takes less than 30 minutes. Results come back within one to two weeks. If the mole turns out to be benign, no further treatment is needed. If melanoma is confirmed, the stage and depth of the tumor determine what comes next.
Why Early Detection Changes Everything
Melanoma that’s still confined to the skin where it started (stages I and II) has a five-year survival rate of about 98%. Once it spreads to nearby lymph nodes (stage III), that drops to around 60%. If it reaches distant organs like the lungs or brain (stage IV), survival falls to roughly 16%. The difference between these numbers is largely a matter of timing. A mole that gets flagged and biopsied early is almost always treatable. One that’s ignored for months or years has a much harder path.
This is why the “E” in ABCDE may be the most important letter. A mole that’s changing is a mole that needs attention, regardless of whether it checks every other box.