Melanoma is the most dangerous form of skin cancer, developing when the pigment-producing cells in your skin (called melanocytes) undergo DNA damage and begin growing out of control. About 90% of melanomas are caused by ultraviolet radiation from sunlight or tanning beds, which mutates the DNA in these cells and disables the body’s natural tumor-suppressing genes. Roughly 332,000 people are diagnosed with melanoma globally each year, and about 58,700 die from it. The good news: when caught early and still confined to the skin, the five-year survival rate is effectively 100%.
How Melanoma Develops
Melanocytes sit in the deepest layer of your outer skin. Their normal job is producing melanin, the pigment that gives skin its color and provides some UV protection. When UV radiation hits these cells repeatedly, it can cause irreversible DNA damage. The damaged cells lose the ability to regulate their own growth, multiply abnormally, and can eventually invade deeper tissues or spread to other organs.
Ironically, the very process of making melanin can generate molecules that cause additional DNA damage, creating a cycle that pushes cells further toward becoming cancerous. This is one reason why even a single severe sunburn, especially in childhood, raises melanoma risk for life.
The ABCDE Rule for Spotting Melanoma
The most widely used tool for identifying a suspicious mole is the ABCDE checklist, developed by the National Cancer Institute. Each letter flags a visual warning sign:
- Asymmetry: One half of the mole doesn’t match the other. Normal moles tend to be roughly symmetrical.
- Border irregularity: The edges are ragged, notched, or blurred rather than smooth. Pigment may appear to bleed into the surrounding skin.
- Color variation: Instead of a single uniform shade, you see a mix of brown, black, tan, 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 start smaller.
- Evolving: The mole has changed in size, shape, color, or texture over recent weeks or months. Any visible change in a mole is worth getting checked.
Not every melanoma checks all five boxes, and some normal moles may hit one or two criteria. The combination of multiple signs, or any single dramatic change, is what should prompt a closer look.
The Ugly Duckling Sign
Beyond ABCDE, there’s a simpler instinct worth trusting: look for the mole that doesn’t match the others. Most people’s moles share a general family resemblance in size, shape, and color. A mole that stands out as obviously different from the rest, the “ugly duckling,” deserves attention even if it doesn’t clearly fit the ABCDE criteria. This comparative approach is particularly useful because you’re the person most familiar with your own skin.
What Different Types Look Like
Melanoma doesn’t always look like a dark mole. Its appearance varies depending on the subtype.
Superficial Spreading Melanoma
This is the most common type. It usually appears as a flat or slightly raised brown patch with irregular, asymmetric borders. You may see areas of black, blue, or pink mixed in. These lesions are typically wider than 6 millimeters and tend to grow outward across the skin surface before pushing deeper.
Nodular Melanoma
Nodular melanoma grows vertically into the skin rather than spreading across it, making it more aggressive from the start. It typically presents as a raised, dome-shaped bump that’s dark brown to black. About 5% of nodular melanomas are “amelanotic,” meaning they have little or no pigment and can look like a flesh-colored or reddish bump, making them easy to dismiss as a pimple or blood blister.
Lentigo Maligna Melanoma
This type develops on chronically sun-damaged skin, most often the face, ears, or arms of older adults. It starts as a large, flat, tan or brown patch that may be 3 centimeters or wider. When it becomes invasive, darker brown-to-black areas or raised blue-black nodules appear within the patch.
Acral Lentiginous Melanoma
This subtype appears on the palms, soles, or under fingernails and toenails, areas not typically associated with sun damage. On palms and soles, it often shows irregular patches of brown, black, and gray pigment. Under a nail, it typically appears as a brown or black vertical stripe running the length of the nail. Advanced cases may show pigment spreading onto the skin around the nail (known as the Hutchinson sign). Acral melanoma is frequently misdiagnosed as a fungal infection, wart, or foot ulcer, which delays treatment. It’s the most common melanoma subtype in people with darker skin tones.
Who Is Most at Risk
Fair-skinned people who burn easily and rarely tan face the highest risk. Skin that always burns and never tans (known as type I skin) is most vulnerable, but melanoma occurs across all skin tones. UV exposure from sunlight or tanning beds is the dominant risk factor for roughly 90% of cases.
Family history matters significantly. Having a first-degree relative (parent, sibling, or child) with melanoma roughly doubles your lifetime risk. If that relative has had multiple melanomas, the risk climbs sharply, up to 16 times higher if they’ve had five or more. Certain inherited gene variants play a role here, particularly those affecting the body’s ability to repair UV-damaged DNA or regulate cell growth. A gene variant associated with red hair and fair skin independently increases melanoma risk even beyond what skin color alone would predict.
Having a large number of moles (typically more than 50), a history of severe sunburns, or a weakened immune system also raises risk.
Why Thickness Determines Severity
When a melanoma is removed, pathologists measure how deep it extends into the skin in millimeters. This measurement, called Breslow thickness, is the single most important factor in determining stage and outlook.
The key depth thresholds are 1.0 mm, 2.0 mm, and 4.0 mm. A melanoma less than 0.8 mm deep without surface ulceration is classified as Stage IA, the earliest and most treatable category. Between 1.0 and 2.0 mm, the stage moves to IIA. Beyond 4.0 mm, particularly with ulceration (where the skin surface over the tumor has broken down), the melanoma reaches Stage IIC. Whether the surface is ulcerated also shifts staging at every depth, because ulceration signals more aggressive behavior.
The survival numbers tell the story of why early detection is so critical. Melanoma caught while still localized to the skin has a five-year survival rate of 100%. Once it spreads to nearby lymph nodes (about 10% of cases at diagnosis), survival drops to roughly 76%. If it reaches distant organs like the lungs, liver, or brain (about 5% of cases), the five-year survival rate falls to around 35%.
How to Check Your Own Skin
A thorough self-exam means checking your entire body from head to toe, including your scalp, the spaces between your toes, the soles of your feet, your nail beds, and your genital area. Use a full-length mirror and a hand mirror to see your back, the backs of your legs, and other hard-to-view spots. A monthly check is a reasonable cadence for most people, though those with significant risk factors may want to check more often.
Pay attention to any new mole that appears after age 30 (new moles become less common with age, so a brand-new spot is more notable), any existing mole that changes shape or color, and any sore that won’t heal. Photographing moles you want to monitor can help you spot subtle changes over time that your memory alone might miss.