Melanoma is a type of skin cancer that develops in melanocytes, the cells responsible for producing the pigment that gives your skin, hair, and eyes their color. While it accounts for a small fraction of all skin cancer diagnoses, melanoma is the most dangerous form because of its ability to spread quickly to other parts of the body. About 90% of melanomas appear on the skin, but they can also develop in the eyes, mouth, nasal passages, and other internal surfaces where melanocytes are present.
How Melanoma Develops
Melanocytes sit in the outer layer of your skin and produce melanin, the pigment that darkens your skin in response to sun exposure. When DNA damage causes these cells to grow out of control, a melanoma forms. The transformation from a normal melanocyte to a cancerous one typically involves mutations in specific signaling pathways that regulate cell growth. Once these pathways get stuck in the “on” position, the cell divides without the normal checks that would slow it down, eventually forming a tumor and triggering the growth of new blood vessels to feed it.
The most common genetic mutation found in melanoma affects a gene called BRAF, present in roughly 38.5% of patients. Another 16.4% carry mutations in the NRAS gene, and about 10% have changes in a gene called KIT. These mutations are mutually exclusive, meaning a melanoma typically carries one or another, not multiple at once. This matters because targeted treatments now exist for some of these specific mutations, particularly BRAF.
The Four Major Subtypes
Melanoma is categorized into four major subtypes based on where in the body it originates.
Cutaneous melanoma is by far the most common, making up about 90% of all diagnosed cases. It develops on regular, hair-bearing skin anywhere on the body and is the type most strongly linked to UV exposure and sun damage.
Acral melanoma appears on hairless skin: the palms of the hands, the soles of the feet, or under the nails. This subtype is not strongly associated with sun exposure and is the most common form of melanoma diagnosed in people with darker skin tones.
Ocular melanoma develops from melanocytes inside the eye, most often in the middle layer of the eye wall called the uveal tract. It is the most common primary cancer that starts inside the eye in adults and behaves quite differently from skin melanoma in terms of its genetic profile and how it spreads.
Mucosal melanoma is rare and aggressive, accounting for less than 1% of all melanomas. It forms in the moist tissue lining internal surfaces of the body, most commonly in the nasal passages and sinuses (31% to 51% of mucosal cases), the anus and rectum (17% to 24%), or the vulva and vagina (about 18%). Because these locations are hidden from view, mucosal melanoma is often diagnosed at a more advanced stage.
What Raises Your Risk
Ultraviolet radiation from the sun is the single biggest environmental risk factor for cutaneous melanoma. UVB rays, the type that cause sunburns and blistering, are considered the primary driver. People who experienced severe sunburns, particularly in childhood, face a meaningfully higher risk. UVA rays also contribute to skin damage and can pass through glass and clouds, which is why cumulative sun exposure matters even on overcast days. Tanning beds emit harmful UVA radiation and carry the same risk.
Your physical traits influence your baseline risk. Fair skin, blue or light-colored eyes, and blond or red hair are all associated with higher rates of melanoma. Skin that freckles easily or burns rather than tans is another indicator. People who work outdoors, spend significant time at the beach, or play outdoor sports accumulate more UV exposure over their lifetimes.
How to Spot It Early
The ABCDE rule is the standard screening tool for identifying suspicious moles or spots on your skin:
- Asymmetry: one half of the mole doesn’t match the other.
- Border: the edges are ragged, notched, or blurred, and pigment may spread into the surrounding skin.
- Color: the mole contains uneven shades of brown, black, or tan, sometimes with patches of white, gray, red, pink, or blue.
- Diameter: the spot is larger than 6 millimeters (about the size of a pencil eraser), though melanomas can start smaller.
- Evolving: the mole has changed in size, shape, or color over weeks or months.
Any single one of these features is worth getting checked. A changing mole is particularly significant, even if it doesn’t tick every other box.
How Depth Determines Stage
When a melanoma is biopsied, one of the most important measurements is its thickness, known as Breslow depth. This number, measured in millimeters, tells doctors how far the cancer has grown downward into the skin and is the primary factor in determining stage and treatment.
Melanomas thinner than 1 millimeter are generally considered early stage. Those between 1 and 2 millimeters are intermediate, and anything thicker than 2 millimeters is classified as a higher-risk tumor. The thicker the melanoma, the greater the chance it has sent cells into nearby lymph nodes or beyond. This depth measurement directly determines how much surrounding tissue a surgeon will remove: thinner melanomas need a 1-centimeter margin of healthy skin around the excision site, while tumors 2 millimeters or thicker require a 2-centimeter margin.
Survival Rates by Stage
Melanoma caught early is highly treatable. The five-year survival rate for localized melanoma (stages I and II, where the cancer has not spread beyond the original site) is 97.6%. That number drops substantially once the cancer reaches nearby lymph nodes: stage III melanoma has a five-year survival rate of 60.3%. For stage IV melanoma, where the cancer has spread to distant organs like the lungs, liver, or brain, the rate falls to 16.2%.
These numbers underscore why early detection matters so much. A melanoma caught at the surface of the skin, before it has a chance to grow deep or spread, is one of the most curable cancers. The same disease diagnosed late becomes one of the most difficult to treat, though newer immunotherapy and targeted therapy options have improved outcomes for advanced melanoma significantly over the past decade.