Melanoma is a type of skin cancer that develops in melanocytes, the cells responsible for producing pigment in your skin. It accounts for a small fraction of all skin cancers but causes the majority of skin cancer deaths because of its ability to spread to other organs. About 2.2 percent of Americans will be diagnosed with melanoma at some point in their lifetime, and catching it early makes an enormous difference: the five-year survival rate for melanoma found before it spreads is 97.6 percent, compared to just 16.2 percent once it reaches distant organs.
How Melanoma Develops
Melanocytes sit in the outer layer of your skin and produce melanin, the pigment that gives skin its color and provides some protection from the sun. When ultraviolet radiation damages the DNA inside these cells, it can trigger mutations that cause the cells to grow out of control. UV exposure is responsible for the highest rate of mutations found in melanoma.
Though melanocytes are most concentrated in the skin, they also exist in the eyes, the lining of the mouth, the intestines, and other internal surfaces. This is why melanoma can occasionally appear in places that never see sunlight, though the vast majority of cases occur on sun-exposed skin.
The ABCDE Rule for Spotting Melanoma
The most widely used framework for recognizing a suspicious mole is the ABCDE rule, developed by the National Cancer Institute. Each letter describes a visual feature that distinguishes melanoma from a normal mole:
- Asymmetry: One half of the spot doesn’t match the other. Normal moles tend to be roughly symmetrical.
- Border: The edges are ragged, notched, or blurred rather than smooth. Pigment may spread into the surrounding skin.
- Color: Instead of a single uniform shade, the spot contains a mix of brown, tan, black, or even areas of white, gray, red, pink, or blue.
- Diameter: Most melanomas are larger than 6 millimeters across (about the size of a pencil eraser), though they can be smaller when first detected.
- Evolving: The mole has changed in size, shape, or color over the past few weeks or months. Any visible change in an existing mole is worth noting.
Not every melanoma checks all five boxes, and not every mole that meets one criterion is cancerous. But any spot that hits two or more of these features deserves a closer look from a dermatologist.
The Ugly Duckling Sign
The ABCDE rule works well for evaluating a single mole, but it has limitations. Some melanomas don’t look particularly unusual on their own. That’s where the “ugly duckling sign” comes in: instead of analyzing one spot in isolation, you compare it to the moles around it. The spot that looks different from all your other moles is the most suspect. Most people’s moles share a general family resemblance in color, size, and shape. The outlier, the one that doesn’t fit the pattern, is the one to get checked.
What the Four Main Types Look Like
Superficial Spreading Melanoma
This is by far the most common type, accounting for roughly 70 percent of all melanomas. It typically appears as a flat or slightly raised patch with irregular borders and uneven color. It tends to grow outward across the skin surface before growing deeper, which is one reason it’s often caught at an earlier, more treatable stage. It can develop anywhere on the body but is most common on the trunk in men and the legs in women.
Nodular Melanoma
The second most common type at 14 to 15 percent of cases, nodular melanoma looks distinctly different. Rather than spreading across the surface, it grows as a raised bump or dome, often dark brown or black, though it can sometimes be red or skin-colored. Because it grows downward into the skin quickly rather than outward, it tends to be thicker at diagnosis and is considered the most aggressive subtype.
Lentigo Maligna Melanoma
Making up 4 to 15 percent of melanomas, this type usually appears on chronically sun-damaged skin in older adults, particularly on the face, ears, and forearms. It starts as a flat, irregularly shaped tan or brown patch that can look like a large age spot. It grows slowly over years, which sometimes causes people to dismiss it as a harmless sun spot.
Acral Lentiginous Melanoma
This is the rarest subtype at about 3 percent of all diagnoses, but it’s especially important to know about because it appears in places people rarely check: the palms of the hands, the soles of the feet, and under the nails. It’s the most common type of melanoma in people with darker skin tones, partly because other subtypes (which favor sun-exposed skin) occur less frequently in this population. On the soles or palms, it looks like an irregularly shaped dark patch.
Melanoma Under the Nails
About 65 percent of nail melanomas first show up as a dark vertical streak running from the base of the nail to the tip, affecting a single nail. These streaks often exceed 3 millimeters in width and may widen as they approach the cuticle. The nail itself may crack, thin, or become distorted alongside the pigment change.
One particularly telling sign is called the Hutchinson sign: pigment that extends beyond the nail onto the surrounding skin of the cuticle or the sides of the nail fold. If you notice a dark brown or black streak on one nail, especially one that’s widening or accompanied by nail damage, that warrants prompt evaluation. This type is frequently diagnosed late because people assume it’s a bruise or fungal infection.
Melanomas That Don’t Look Like Melanomas
Not all melanomas are dark. Amelanotic melanoma produces little to no pigment and often appears as a pink to red spot, bump, or raised nodule. Because it doesn’t match the classic image of a dark, irregular mole, it’s easily mistaken for a pimple, bug bite, or scar. These account for a small percentage of cases, but they’re disproportionately likely to be caught late precisely because they don’t trigger the visual alarms people associate with skin cancer. Any persistent pink or red bump that doesn’t heal within a few weeks is worth showing to a doctor.
Risk Factors That Matter Most
UV exposure is the dominant modifiable risk factor. Childhood sunburns are particularly damaging and may double the risk of developing melanoma later in life. But it’s not just childhood exposure. The total number of sunburns accumulated across a lifetime also correlates with risk, meaning sun protection matters at every age.
Indoor tanning carries its own serious risk. Tanning beds emit UVA radiation at doses up to 12 times what you’d receive from the sun. Using indoor tanning before age 30 increases melanoma risk by 75 percent, and the risk climbs further with more frequent use regardless of age.
Other risk factors include having fair skin that burns easily, a large number of moles (especially more than 50), a family history of melanoma, a personal history of any skin cancer, and a weakened immune system. Having one or more of these doesn’t mean melanoma is inevitable, but it does mean regular skin checks become more important.
Why Thickness Determines the Outlook
When melanoma is diagnosed, the single most important measurement is called the Breslow depth: how deep the tumor has grown into the skin, measured in millimeters from the surface. Thinner melanomas have dramatically better outcomes. A melanoma less than 1 millimeter deep has a very different prognosis than one that’s 4 millimeters deep, because depth correlates directly with the likelihood that cancer cells have reached blood vessels or lymph nodes.
The good news is that about 83 percent of melanomas are diagnosed while still localized to the skin, before they’ve spread. At that stage, the five-year survival rate is nearly 98 percent. Once melanoma reaches nearby lymph nodes, that rate drops to about 60 percent. This gap is the entire reason early detection matters so much: the difference between a thin, localized melanoma and one that has begun to spread is often the difference between a straightforward surgical removal and a far more intensive treatment course.
How to Check Your Own Skin
A monthly self-exam takes about 10 minutes and requires a full-length mirror and a hand mirror. Start with your face, ears, and scalp (use a comb or hair dryer to part your hair). Move to your arms, including the undersides and between your fingers. Check your chest, abdomen, and back using the hand mirror. Examine your legs front and back, and don’t skip the soles of your feet, between your toes, and your toenails.
You’re looking for anything new, anything that’s changed, and anything that stands out from the surrounding moles. Take photos of spots you want to track so you can compare them month to month. Changes in size, shape, or color over weeks to months are more concerning than a spot that’s looked the same for years.