Skin cancer starts when ultraviolet radiation damages the DNA inside skin cells, and the body’s normal repair systems fail to fix it. Over time, these unrepaired mutations accumulate, causing cells to grow uncontrollably and form tumors. It’s the most common cancer in humans, and understanding the step-by-step process helps explain why sun protection matters so much and who faces the greatest risk.
How UV Light Damages Your DNA
Sunlight contains two types of ultraviolet radiation that harm your skin in different ways. UVB rays, the ones responsible for sunburn, don’t penetrate very deep but cause direct damage to DNA in the outer skin layers. They force neighboring building blocks in your DNA strand to fuse together, creating defects called pyrimidine dimers. These fused spots distort the shape of your DNA and can cause errors when the cell tries to copy itself.
UVA rays have a longer wavelength and reach deeper into the skin, damaging the structural proteins that keep skin firm while also generating unstable molecules called free radicals. These free radicals can attack DNA indirectly. UVA partners with UVB to amplify the overall cancer risk, which is why both tanning beds and natural sunlight are dangerous. The damage isn’t always immediate or visible. Much of it accumulates silently over years of exposure.
When a cell copies its damaged DNA, it sometimes reads the fused spot incorrectly and inserts the wrong genetic letter. Research from Nucleic Acids Research shows that these dimers become especially prone to causing mutations after a chemical change occurs at the damage site, converting one DNA letter into another. The copying machinery then locks in a permanent error. This is the moment a normal gene can become a cancer-promoting one.
What Happens When DNA Repair Fails
Your cells have a built-in safety system designed to catch and fix DNA errors before they become permanent. The central player is a protein known as p53, often called the “guardian of the genome.” When p53 detects serious DNA damage, it can pause cell division to allow time for repairs, activate specialized repair crews, or, if the damage is too severe, trigger the cell to self-destruct entirely. This self-destruction prevents a dangerously mutated cell from surviving and multiplying.
The problem is that UV radiation can mutate the gene for p53 itself. When p53 stops working, the entire quality-control system breaks down. Damaged cells no longer get repaired or eliminated. They keep dividing, passing their mutations to daughter cells, which accumulate even more errors with each round of copying. The p53 pathway is impaired in the vast majority of non-melanoma skin cancers, making it one of the most important single points of failure in skin carcinogenesis.
From Mutation to Uncontrolled Growth
A single mutation rarely causes cancer on its own. Skin cancer typically requires multiple genetic hits that accumulate over time. One mutation might disable p53. Another might activate a growth-signaling gene. Together, these changes override the normal checks that keep cell division orderly.
In melanoma, the most dangerous form of skin cancer, the key mutations often strike genes called BRAF and NRAS. These genes are part of a signaling chain that tells cells when to grow and divide. When BRAF or NRAS is permanently switched “on” by a mutation, the cell receives a constant growth signal it can’t turn off. A majority of cutaneous melanomas carry an activating mutation in one of these two genes, and the mutations appear early in the cancer’s development, acting as a kind of molecular starting pistol.
For squamous cell and basal cell carcinomas, the process involves different cells and slightly different genetic pathways, but the principle is the same: accumulated mutations remove the brakes on cell growth while simultaneously jamming the accelerator.
Where Each Type of Skin Cancer Begins
The three major types of skin cancer originate from different cells in the skin, which is why they look and behave so differently.
- Basal cell carcinoma arises most frequently from cells in the interfollicular epidermis, the flat stretches of your outermost skin layer between hair follicles. It’s the most common and slowest-growing type, rarely spreading to distant organs but capable of destroying surrounding tissue if ignored.
- Squamous cell carcinoma can originate from multiple cell populations, including hair follicle stem cells and cells in the outer epidermis. Research in PNAS demonstrated that squamous tumor formation isn’t restricted to one skin cell lineage, but it does require multiple genetic hits beyond the initial activating mutation to become invasive.
- Melanoma starts in melanocytes, the pigment-producing cells scattered through the base of the epidermis. Because melanocytes are connected to the body’s pigment and immune signaling networks, melanoma is more likely to spread aggressively to other organs.
The Precancerous Stage
Skin cancer doesn’t always appear out of nowhere. Squamous cell carcinoma, in particular, often has a recognizable precursor: rough, scaly patches called actinic keratoses that develop on sun-exposed skin. These patches represent cells that have already accumulated significant DNA damage but haven’t yet crossed the line into invasive cancer.
The risk that any single actinic keratosis will progress to invasive squamous cell carcinoma ranges from about 0.025% to 16% per year depending on the study, with an average estimate around 8% across reviewed data. That per-lesion number sounds low, but many people with sun-damaged skin have dozens of these patches, and the cumulative risk across all of them adds up. Doctors typically treat actinic keratoses based on factors like how long the patch has persisted, the patient’s age, overall sun damage, and personal history of skin cancer.
Why Some People Are More Vulnerable
Genetics play a major role in determining how well your skin can defend itself against UV damage. The pigment melanin acts as a natural sunscreen, absorbing UV photons before they reach the DNA inside your cells. People with darker skin produce more of a type of melanin called eumelanin, which is highly effective at this job. People with lighter skin, especially those with red hair and freckles, tend to produce more pheomelanin, a less protective form.
The gene most responsible for this difference is MC1R, which controls how melanocytes respond to signals telling them to produce pigment. Certain variants of MC1R reduce the receptor’s ability to function, shifting pigment production toward pheomelanin. A study in The American Journal of Human Genetics found that these MC1R variants increase the risk of non-melanoma skin cancer independently of just having fair skin and red hair. In other words, the gene itself raises risk through mechanisms beyond simple pigment color, possibly by affecting how melanocytes handle DNA repair and oxidative stress.
Causes Beyond Sunlight
While UV radiation drives the overwhelming majority of skin cancers, it isn’t the only cause. Chronic exposure to arsenic, primarily through contaminated drinking water, has been linked to increased skin cancer risk. Research at the University of Arizona examined arsenic levels in toenail samples (a marker of long-term exposure) to study its role in melanoma development. Arsenic is believed to interfere with DNA repair processes, compounding the effects of other risk factors.
Other non-UV causes include exposure to ionizing radiation (such as from repeated medical radiation treatments to the same area), chronic wounds or burns that never fully heal, and long-term immune suppression. Organ transplant recipients, for example, face dramatically higher rates of squamous cell carcinoma because their anti-rejection medications weaken the immune system’s ability to detect and destroy abnormal cells. In each of these cases, the underlying mechanism is the same: something damages DNA or prevents the body from eliminating cells with damaged DNA, and mutations accumulate until growth becomes uncontrollable.