Skin cancer is the uncontrolled growth of abnormal cells in the outermost layer of skin, triggered most often by DNA damage from ultraviolet radiation. It is the most common type of cancer in the United States, with melanoma alone accounting for an estimated 104,960 new cases in 2025. There are three major types, each named for the skin cell where it starts: basal cell carcinoma, squamous cell carcinoma, and melanoma.
How UV Radiation Causes Skin Cancer
Ultraviolet light from the sun or tanning beds is the primary driver of skin cancer. When UV rays penetrate your skin, they physically damage the DNA inside your cells, creating abnormal links between DNA building blocks called thymine dimers. These kinks in the DNA strand are normally repaired by your body’s built-in repair machinery. But when the damage outpaces repair, or when the repair system itself is compromised, mutations accumulate.
The mutations that matter most tend to hit genes controlling cell growth and cell death. The p53 gene, which normally acts as a brake on damaged cells by telling them to stop dividing or self-destruct, is the most frequent target of UV-induced mutations. When p53 stops working, damaged cells survive and keep multiplying instead of being eliminated. Over years or decades, that unchecked growth becomes a tumor.
Both UVA and UVB rays contribute. UVB causes the most direct DNA damage, while UVA penetrates deeper into the skin and generates reactive molecules that damage DNA indirectly. This is why broad-spectrum sun protection, covering both wavelengths, matters.
Basal Cell Carcinoma
Basal cell carcinoma (BCC) is the most common skin cancer by a wide margin. It starts in the basal cells at the bottom of the outer skin layer and almost always appears on sun-exposed areas, particularly the face. The nose, cheeks, forehead, and eyelids are the most frequent sites.
A typical BCC looks like a shiny, pink or flesh-colored bump with a pearly quality and tiny visible blood vessels on its surface. As it grows, the center may ulcerate and develop raised, rolled borders, sometimes called a “rodent ulcer.” Not all BCCs look the same, though. Superficial BCCs appear as flat, pink-to-red scaly patches, often on the chest, shoulders, or back. Another variant resembles a white or flesh-colored scar with poorly defined edges, making it easy to overlook.
BCC grows slowly and very rarely spreads to other parts of the body. The real danger is local tissue destruction. Left untreated, a BCC on the nose or near the eye can erode through cartilage, bone, or other structures, causing significant disfigurement. Early removal is straightforward and highly effective.
Squamous Cell Carcinoma
Squamous cell carcinoma (SCC) arises from the flat cells that make up most of the skin’s outer surface. It is the second most common skin cancer and carries a somewhat higher risk of spreading than BCC, particularly when it develops on the lips, ears, or in people with weakened immune systems.
SCC can take several forms: a firm, skin-colored or reddish nodule, a flat sore with a scaly crust, a rough patch on the lip that may become an open sore, or a new raised area on an old scar. It can also develop inside the mouth or on the genitals. One important detail is that SCC often has a precursor. Rough, scaly patches called actinic keratoses, caused by cumulative sun exposure, can transform into squamous cell carcinoma over time. Having these precancerous spots increases your risk, and treating them early can prevent progression.
Melanoma
Melanoma develops in the pigment-producing cells called melanocytes. It accounts for a small fraction of all skin cancers but causes the majority of skin cancer deaths because of its ability to spread quickly to lymph nodes and distant organs. An estimated 8,430 Americans will die from melanoma in 2025.
The good news is that melanoma caught early, while still confined to the skin, has an excellent prognosis. The challenge is recognizing it. The ABCDE rule, first introduced in 1985 and expanded over the following decades, provides a practical framework for spotting suspicious moles:
- Asymmetry: One half of the mole doesn’t match the other.
- Border: The edges are irregular, ragged, or blurred.
- Color: The mole contains multiple shades of brown, black, red, white, or blue rather than a single uniform color.
- Diameter: The spot is larger than 6 millimeters, roughly the size of a pencil eraser.
- Evolving: The mole is new or changing in size, shape, or color.
There’s also a complementary approach called the “ugly duckling sign.” Rather than evaluating a single mole against a checklist, you look at the overall pattern of your moles. The one that looks different from all the others, the outlier, is the most suspect for melanoma. This is especially useful for people with many moles, where individual features can be harder to assess.
Less Common Types
Merkel cell carcinoma is a rare but aggressive skin cancer that typically appears as a painless, firm bump on sun-exposed skin. It occurs most often in people over 50. In white patients, it tends to develop on the head or neck; in Black patients, it more commonly appears on the legs. Because it grows and spreads quickly, early detection is critical.
Other rare skin cancers include dermatofibrosarcoma protuberans, a slow-growing tumor in the deeper layers of skin, and sebaceous carcinoma, which typically develops on the eyelid. These are uncommon enough that most people will never encounter them, but any new, persistent, or changing skin growth warrants attention.
Who Is Most at Risk
Your natural skin color is one of the strongest predictors of skin cancer risk. Dermatologists classify skin into six phototypes based on how it responds to sun exposure. People with Type I skin (very fair, always burns, never tans, often with blue eyes and fair hair) face the highest risk. Those with Type IV skin (light brown, rarely burns, tans easily) have substantially lower but not zero risk. People with the darkest skin tones (Types V and VI) develop skin cancer least often, but when they do, it tends to be diagnosed at a later stage, partly because neither patients nor providers expect it.
Beyond skin type, other risk factors include a history of sunburns (especially blistering burns in childhood), frequent use of tanning beds, a weakened immune system from medications or illness, a personal or family history of skin cancer, and having many moles. Cumulative sun exposure over a lifetime matters more for BCC and SCC, while intense, intermittent burns are more strongly linked to melanoma.
How Skin Cancer Is Diagnosed
Diagnosis starts with a visual examination, but confirmation always requires a biopsy, where a small sample of tissue is removed and examined under a microscope. The type of biopsy depends on the suspected cancer.
For suspected melanoma, an excisional biopsy is preferred whenever the size and location allow. This removes the entire visible lesion so a pathologist can assess its full depth, which is essential for determining how advanced it is. For suspected non-melanoma skin cancers, a shave biopsy (scooping a thin layer of tissue with a blade) is often sufficient. A punch biopsy uses a small cylindrical tool to take a full-thickness core of skin and is useful when deeper tissue needs to be evaluated or when a complete excision isn’t practical.
Treatment Options
Treatment depends on the type, size, location, and stage of the cancer. For most BCCs and SCCs, surgical removal is the primary approach and is often curative.
For cancers in cosmetically sensitive areas like the face, or tumors with aggressive features, a specialized technique called Mohs surgery offers the highest cure rates. During this procedure, the surgeon removes a thin layer of tissue and immediately examines it under a microscope, mapping exactly where cancer cells remain. Additional layers are removed only where cancer is found, sparing as much healthy tissue as possible. The process of removing a layer, examining it, and going back for more takes cycles of 20 minutes to an hour each, and patients wait between rounds. Once all margins are clear, the wound is repaired, often the same day. This approach is the standard of care for high-risk basal cell and squamous cell carcinomas.
Melanoma treatment depends heavily on depth. Thin melanomas may need only surgical excision with a margin of healthy skin. Deeper melanomas often require a sentinel lymph node biopsy to check for spread, and advanced melanoma may be treated with immunotherapy or targeted therapy drugs that have dramatically improved survival over the past decade.
Prevention and Early Detection
The American Academy of Dermatology recommends sunscreen with an SPF of 30 or higher that provides broad-spectrum protection against both UVA and UVB rays. Most adults need about one ounce, roughly enough to fill a shot glass, to cover all exposed skin. For the face alone, use at least one teaspoon. Reapply every two hours, and immediately after swimming or sweating. A lip balm with SPF 30 or higher protects an area often overlooked.
Sunscreen is one layer of a broader strategy. Seeking shade during peak UV hours (roughly 10 a.m. to 4 p.m.), wearing sun-protective clothing, wide-brimmed hats, and UV-blocking sunglasses all reduce your cumulative exposure. Avoiding tanning beds eliminates a significant and entirely preventable source of UV damage.
For early detection, monthly self-exams help you learn the map of your own skin so that changes stand out. Check everywhere, including the scalp, between toes, and the soles of your feet. Use the ABCDE criteria and the ugly duckling sign to evaluate moles, and pay attention to any sore that doesn’t heal within a few weeks. Annual skin checks with a dermatologist are particularly valuable if you have risk factors like fair skin, a history of sunburns, or a family history of melanoma.