Melanoma is caused by damage to the DNA inside melanocytes, the cells that give your skin its color. That damage most often comes from ultraviolet (UV) radiation, but genetics, immune system function, and even physical trauma can play a role. Understanding what triggers this damage helps explain why some people develop melanoma and others don’t, even with similar sun exposure.
How UV Radiation Damages Skin Cells
UV radiation from the sun is the single biggest cause of melanoma. When UV light hits your skin, it’s absorbed directly by the DNA inside your cells, distorting the chemical bonds that hold genetic information together. This creates errors called “UV signature mutations,” and if the cell can’t repair them fast enough, the mutations accumulate and can eventually turn a normal melanocyte cancerous.
There are two types of UV radiation that matter here, and they cause damage differently. UVB, the type responsible for sunburns, penetrates the outer layer of skin and directly creates structural defects in DNA. About 80% of these defects are a specific type of lesion that, if left unrepaired, produces the characteristic mutations found in most melanomas. UVA penetrates deeper and works indirectly. It excites molecules inside the cell, including melanin itself, generating reactive oxygen species that attack DNA from within. Melanocytes are especially vulnerable to this because they produce melanin as part of their normal function, meaning the very pigment meant to protect your skin can become a source of internal damage when hit with UVA light.
One particularly striking finding: both UVA and UVB can trigger something called “dark” DNA damage in melanocytes, where mutations continue forming for hours after sun exposure has ended. This means the damage doesn’t stop the moment you step out of the sun.
Sunburns and Tanning Beds
Not all UV exposure carries equal risk. Intense, intermittent exposure, the kind that causes blistering sunburns, is more strongly linked to melanoma than steady, low-level exposure. Childhood sunburns are particularly dangerous. Research estimates that painful or blistering sunburns before age 15 increase melanoma risk by roughly 50% across different body sites, with some analyses suggesting the causal effect could be several times higher.
Indoor tanning dramatically amplifies risk. Using a tanning bed before age 35 is associated with a 75% increased risk of developing melanoma. Tanning beds emit concentrated UV radiation, often with high levels of UVA, which penetrates deep into the skin and generates the oxidative damage melanocytes are uniquely susceptible to.
Skin Type and Mole Count
Your baseline risk depends partly on physical characteristics you’re born with. People with very fair skin that burns easily and rarely tans (skin types 1 and 2 on the Fitzpatrick scale) face the highest risk because they have less natural melanin to absorb UV before it reaches cell DNA. But melanoma can develop in people with any skin tone, including very dark skin, so pigmentation is protective, not preventive.
The number and type of moles on your body is one of the strongest predictors of melanoma risk. Having even one atypical mole (a mole that’s irregular in shape, color, or size) raises your risk by about 60% compared to someone with none. Having five or more atypical moles increases risk roughly 6 to 10 times, even after accounting for sun exposure, family history, and other factors. Most moles never become cancerous, but a high count signals that your melanocytes are more prone to abnormal growth.
Genetic Mutations That Drive Melanoma
At the molecular level, melanoma is driven by mutations in specific genes that control cell growth. The most common is BRAF, mutated in about 38.5% of melanoma cases. NRAS mutations account for another 16.4%, and KIT mutations are found in about 10%. These mutations essentially jam the cell’s growth signals in the “on” position, causing melanocytes to multiply uncontrollably.
Some people inherit gene mutations that dramatically raise their risk before any sun exposure enters the picture. The most significant is a mutation in the CDKN2A gene, a tumor suppressor that normally acts as a brake on cell division. Inheriting a faulty copy of this gene increases melanoma risk by 65-fold. CDKN2A mutations are found in about 22% of families with a strong history of melanoma, and over half of people who develop multiple separate melanomas carry a mutation in this gene. Several other inherited gene variants also contribute, but CDKN2A is by far the most impactful.
Weakened Immune System
Your immune system constantly patrols for abnormal cells and destroys them before they can form tumors. When that surveillance system is suppressed, melanoma risk rises. This is most clearly seen in organ transplant recipients, who take immunosuppressive medications for life to prevent rejection. Their risk of melanoma increases with both the duration and intensity of immunosuppression. The medications don’t just weaken immune monitoring; some have direct effects that promote cancer cell growth. Older age at the time of transplant, fair skin, and cumulative sun exposure further compound this risk.
Chemical and Environmental Exposures
UV radiation gets the most attention, but chemical exposures also play a role. People who use pesticides regularly have roughly double the risk of developing melanoma compared to those who don’t. Occupational exposure is even more concerning: workers who handle pesticides as part of their job face about four times the risk. Indoor domestic use of pesticides, particularly frequent or long-term use, appears to be an independent risk factor separate from sun exposure. The exact biological mechanism isn’t fully established, but these chemicals can cause DNA damage and disrupt cellular repair processes.
Melanoma in Places the Sun Doesn’t Reach
One of the more puzzling forms of this cancer is acral lentiginous melanoma, which develops on the palms, soles of the feet, and under fingernails or toenails. UV radiation doesn’t appear to play a significant role here. These melanomas lack the UV signature mutations found in other types and instead show a different pattern of genetic changes, most commonly extra copies of the KIT gene (found in up to 36% of cases) and instability in a gene called Cyclin D1 (found in about 45%).
Researchers have proposed that mechanical stress may be a trigger. Acral melanoma occurs more frequently on weight-bearing areas of the foot like the heel and forefoot, and there’s a correlation between penetrative injuries and increased likelihood of developing it. Obese patients show higher rates of melanoma on the arch of the foot, further supporting the mechanical stress theory. The genetic profile of these tumors also varies by ethnicity, suggesting that multiple pathways can lead to the same disease depending on a person’s background.
The cells surrounding these tumors often already carry some of the same mutations as the tumor itself, a phenomenon called “field effect.” This may explain why acral melanomas sometimes recur locally even after the visible lesion is completely removed.
Multiple Causes Working Together
In most cases, melanoma isn’t caused by a single factor. It develops when several risk factors overlap: a genetic predisposition that makes your melanocytes more vulnerable, UV exposure that creates DNA damage faster than your cells can repair it, and an immune system that fails to catch the abnormal cells before they establish a foothold. Someone with fair skin, a family history, dozens of moles, and a history of childhood sunburns faces a very different risk profile than someone with just one of those factors. The combination matters more than any individual cause.