Most skin spots are harmless, but a few key warning signs can help you tell the difference between a normal mole or blemish and something that needs medical attention. The three most common skin cancers, melanoma, basal cell carcinoma, and squamous cell carcinoma, each look different on the skin. Knowing what to watch for across all three gives you the best chance of catching a problem early.
The ABCDE Rule for Melanoma
Melanoma is the most dangerous form of skin cancer, but it’s also the one with the clearest early warning signs. Dermatologists use a five-letter system to describe what suspicious moles look like:
- Asymmetry: One half of the spot doesn’t match the other half.
- Border: The edges are ragged, notched, or blurred rather than smooth and well-defined. Pigment may spread into the surrounding skin.
- Color: The spot isn’t one uniform shade. You might see a mix of brown, black, and tan, or areas of white, gray, red, pink, or blue within the same spot.
- Diameter: Most melanomas are larger than 6 millimeters across (roughly the size of a pencil eraser), though they can be smaller.
- Evolving: The spot has changed in size, shape, or color over the past few weeks or months.
A spot doesn’t need to meet all five criteria to be concerning. Any single one of these features, especially a spot that’s evolving, is worth getting checked.
The Ugly Duckling Sign
The ABCDE rule works well for evaluating a single spot, but there’s a complementary approach that’s just as useful. The “ugly duckling sign” means looking at all your moles as a group and flagging the one that doesn’t look like the others. Most of your moles will share a general family resemblance in terms of size, shape, and color. The outlier, the spot that stands apart from the rest, is the one most likely to be a problem. This method catches melanomas that might not obviously fail any single ABCDE criterion on their own but clearly don’t belong with their neighbors.
What Basal Cell Carcinoma Looks Like
Basal cell carcinoma is the most common skin cancer, and it usually shows up on sun-exposed areas like the head and neck. It doesn’t look like a typical mole. Instead, it appears as a change in the skin that takes one of several forms:
- A shiny, translucent bump that looks pearly white or pink on lighter skin, or brown to glossy black on darker skin. You may be able to see tiny blood vessels on or near it. These bumps can bleed and scab over repeatedly.
- A brown, black, or blue lesion with a slightly raised, translucent border and dark spots within it.
- A flat, scaly patch that may or may not have a raised edge. These can grow quite large over time.
- A white, waxy, scarlike area without a clearly defined border.
The hallmark of basal cell carcinoma is a sore that won’t heal. If you have a spot that bleeds, scabs over, seems to heal, then opens back up again in a cycle that repeats for weeks, that pattern alone warrants a closer look.
What Squamous Cell Carcinoma Looks Like
Squamous cell carcinoma is the second most common skin cancer. It tends to appear as a firm bump (sometimes the same color as surrounding skin, sometimes pink, red, brown, or black), a flat sore with a scaly crust, or a rough, wartlike growth. One distinctive presentation is a rough, scaly patch on the lip that eventually becomes an open sore. It can also develop inside the mouth.
A practical rule: if you have a sore or scab that hasn’t healed in about two months, or a flat, scaly patch that simply won’t go away, that’s a signal to get it evaluated.
Precancerous Rough Patches
Before squamous cell carcinoma develops, you may notice rough, dry, scaly patches on sun-exposed skin. These are called actinic keratoses, and they feel like sandpaper when you run your finger over them. Most individual patches never become cancer. The average risk of any single patch progressing to squamous cell carcinoma is estimated around 8% per year, but that risk climbs in people who have five or more of these patches. Since there’s no reliable way to predict which ones will progress and which won’t, getting them treated early is the simplest approach.
Spots in Places You Wouldn’t Expect
Skin cancer doesn’t only show up on sun-exposed areas. A type of melanoma called acral lentiginous melanoma develops on the soles of the feet, the palms of the hands, and under fingernails or toenails. On the palm or sole, it looks like a black or brown discoloration that might initially resemble a bruise or stain, but it grows in size over time rather than fading.
Under the nails, it appears as dark vertical streaks or discolorations running along the nail bed. As it progresses, it can cause the nail to crack or break. This type is sometimes mistaken for a fungal infection or dried blood under the nail. If a dark streak under your nail isn’t growing out with the nail or has been there for more than a few weeks, it’s worth showing to a professional.
Fast-Growing Bumps Need Urgent Attention
Most skin cancers grow slowly, but some don’t. Merkel cell carcinoma is rare and aggressive. It appears as a painless bump that grows quickly, often over just weeks. The bump may look pink, purple, red-brown, or match the surrounding skin tone, and its two sides typically don’t match. If you notice a bump that’s visibly larger week to week, or that bleeds easily after minor contact like washing or shaving, treat that as urgent.
Spots That Mimic Cancer (and Vice Versa)
Seborrheic keratoses are extremely common benign growths that appear with age. They’re often brown or black, waxy-looking, and slightly raised, and they can look alarming. The classic difference is texture: seborrheic keratoses tend to have a “stuck-on” appearance, as if you could peel them off the skin, and their surface is often rough or crumbly in a uniform way. Melanoma, by contrast, tends to show irregular pigmentation, with color that varies unevenly across the spot, and its borders are more ragged and unpredictable.
The problem is that some melanomas can closely mimic seborrheic keratoses, even to experienced eyes. In studies of melanomas that resembled seborrheic keratoses, nearly 87% had irregular pigmentation patterns that only became apparent under magnification. The takeaway: if a “normal” age spot starts changing, developing new colors, or growing in a way it didn’t before, don’t assume it’s still benign.
How to Do a Self-Check
There’s no universally agreed-upon schedule for skin self-exams. The U.S. Preventive Services Task Force hasn’t found enough evidence to set a firm recommendation either for or against routine self-checks. That said, familiarizing yourself with your own skin is the most practical thing you can do, because noticing change requires knowing what was there before.
A thorough check takes about 10 minutes. Stand in front of a full-length mirror and look at your front and back, then raise your arms and check both sides. Use a hand mirror for your scalp, the backs of your ears, your neck, and your back. Sit down to examine your legs, the tops and soles of your feet, and between your toes. Check your palms, the spaces between your fingers, and under your fingernails. Partners can help with hard-to-see areas like your back and scalp.
The goal isn’t to diagnose anything yourself. It’s to notice something new, different, or changing so you can bring it to a professional before it has time to progress.
What Happens at a Professional Evaluation
If you bring a suspicious spot to a dermatologist, the first step is usually a visual exam, sometimes with a handheld magnifying device called a dermatoscope that illuminates the skin’s deeper layers. If the spot looks concerning, the next step is a biopsy, which means removing a small sample of tissue to examine under a microscope. There are three common types:
- Shave biopsy: A thin layer of skin is shaved off with a blade. No stitches needed.
- Punch biopsy: A small circular tool removes a deeper, round sample. Usually requires a stitch or two.
- Excisional biopsy: The entire spot is cut out with a scalpel, along with a margin of surrounding skin. This also requires stitches.
Which method is used depends on the size, location, and depth of the spot. All three are done under local anesthesia in an office setting and take only minutes. Results typically come back within one to two weeks.