The short answer
A mole or skin spot that changes in size, shape, or color, or a sore that does not heal, can be a sign of skin cancer, including melanoma. The ABCDE rule helps spot warning signs to bring to a doctor.
A mole that changes in size, shape, or color can be a warning sign of skin cancer.
The ABCDE rule helps: Asymmetry, Border, Color, Diameter, and Evolving.
A new growth or a sore that does not heal can also be a sign of skin cancer.
Melanoma is the most serious skin cancer and is very treatable when found early.
Watch: When is a mole worth checking?
1 min 3 sec · Captioned · Normal moles usually don't change — the ABCDE signs worth checking.
Educational only — this video explains general report language and is not medical advice. Only your care team can say what a result means for you.
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The full explanation.
The simple version
Most moles are harmless. But a mole or skin spot that changes over time, or a sore that will not heal, can be a warning sign of skin cancer, including melanoma. The good news: skin cancer found early is very treatable.
The ABCDE rule
The National Cancer Institute uses a five-letter checklist to describe early melanoma:
- A — Asymmetry. The shape of one half does not match the other half.
- B — Border. The edges look ragged, notched, or blurred, instead of smooth.
- C — Color. Shades of black, brown, and tan may show up together. Areas of white, gray, red, pink, or blue can appear too.
- D — Diameter. The spot changes size, usually growing larger. Melanomas are often bigger than 6 millimeters wide, about a quarter inch, roughly the size of a pencil eraser.
- E — Evolving. The mole has changed over the past few weeks or months.
Not every melanoma shows all five signs. Some show only one or two. That is why any one of these changes is worth a doctor's look, not just all five together. A normal mole, by contrast, usually keeps a steady color, a smooth border, and a stable size for years.
Other skin warning signs
Melanoma is not the only skin cancer to watch for. Basal cell and squamous cell cancers are far more common, and they look different from melanoma. Watch for:
- A new growth or bump on the skin that was not there before.
- A sore that does not heal, or that bleeds, oozes, or forms a crust.
- A pearly, waxy, or shiny bump, which can be white, pink, brown, or skin-colored.
- A firm bump, flat sore, or scaly patch that does not settle down.
- A dark band under a fingernail or toenail.
On darker skin, skin cancer often shows up in less expected places and colors. A dark patch on the palm or the sole of the foot, or a dark band under a nail, can be the first sign of melanoma. Basal cell cancer can look like a brown or glossy black bump with a rolled edge, and squamous cell cancer can appear as a firm bump or scaly patch that may look black or brown rather than red. Because these patterns are easy to miss, any unusual, unexplained skin change deserves attention regardless of skin tone.
Why this matters
Skin cancer is one of the more visible cancers, which is both good and bad news. Good, because you and your doctor can often see it directly, without scans or blood tests. Bad, because that means the responsibility often falls on you to notice a change and act on it. A spot that looks different from your other moles, sometimes called the "ugly duckling" sign, is worth checking even if it does not fit the ABCDE pattern exactly.
When to see a doctor
See a doctor or dermatologist about any skin change that does not go away within about a month. This includes a mole that is changing, a new growth, a sore that will not heal, or a nail with a new dark streak. Do not wait for pain. Many skin cancers, including melanoma, cause no pain in their early stages.
At the visit, the doctor examines the spot and may take a biopsy: removing some or all of the tissue so a pathologist can look at it under a microscope. A biopsy is the only way to know for certain whether a spot is cancer. If the diagnosis is uncertain, it is reasonable to ask for the sample to be reviewed by a second pathologist.
What to track before your visit
Take a photo of any spot you are watching, with something for scale, like a coin, next to it. Note when you first noticed it and anything that has changed since. If you are not sure whether something looks different from your other moles, bring it up anyway. Early melanoma is very treatable, so getting a new or changing spot checked is always worth the ten minutes it takes.
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Words to know
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Common questions
What changes in a mole should I watch for?
Watch for a mole that changes in size, shape, or color, or one that looks different from your others. A helpful guide is the ABCDE rule: Asymmetry, an irregular Border, uneven Color, a large Diameter, and Evolving or changing over time.
What is the ABCDE rule?
It is a simple checklist for moles: Asymmetry (one half unlike the other), Border (irregular edges), Color (more than one shade), Diameter (larger than a pencil eraser), and Evolving (changing over time).
What else signals skin cancer?
A new growth, a sore that does not heal, or a spot that itches, bleeds, or crusts can be signs of skin cancer, including basal or squamous cell cancers.
When should I see a doctor?
See a doctor or dermatologist for any mole or skin spot that is changing, looks unusual, or does not heal. Melanoma is very treatable when caught early.
Questions to ask your doctor
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Last updated: 2026-08-05Next planned review: 2027-07-07
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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