What is Skin Cancer?

This is the first article in the Understanding Skin Cancer Guide. This guide was developed by the HealthTree Education Team and was last updated and reviewed on May 14, 2026

Cancer begins when cells in the body start to grow out of control. Skin cancer starts in the cells of the skin, most often in the areas that are frequently exposed to the sun, such as the face, neck, hands, and arms, though it can develop anywhere on the body.

When you have skin cancer, skin cells do not follow the normal cell cycle. Healthy cells grow, copy themselves, and then die. Cancer cells change, grow faster than normal, and do not die the way they should.

What is the skin?

The skin is the body’s largest organ. IT acts as a protective barrier against the environment, regulates body temperature, and helps the body sense touch, heath and cold. The skin has three main layers:

• The epidermis is the outermost layer of skin. It contains several types of cells, including squamous cells (flat cells near the surface), basal cells (round cells beneath the squamous cells), and melanocytes (cells that produce the pigment melanin, which gives skin its color). Most skin cancers begin in the epidermis.
• The dermis is the layer beneath the epidermis. It contains hair follicles, sweat glands, blood vessels, and nerve endings.
• The hypodermis (subcutaneous tissue) is the deepest layer, made mostly of fat and connective tissue.

How skin cancer starts

Most skin cancers develop when UV radiation from the sun or tanning beds damages the DNA in skin cells. Over time, this damage can cause cells to grow out of control. Skin cancer can also develop in areas of the skin that are not regularly exposed to sunlight, in which other factors such as inherited gene mutations, chronic wounds, or immune system problems may play a role.

How does skin cancer affect the body?

As skin cancer grows, it can affect many parts of the body:

How skin cancer affects the skin:

A growing skin cancer can destroy surrounding skin tissue, cause open sores or wounds that do not heal, and change the appearance of the skin. In advanced cases, tumors may grow deep into underlying tissue, muscle, or bone.

How skin cancer affects the lymph nodes:

Skin cancer, particularly melanoma, can spread to nearby lymph nodes. Swollen lymph nodes in the neck, armpit, or groin may be a sign that the cancer has reached the lymphatic system.

How skin cancer affects other organs:

When cancer spreads to other parts of the body, it has metastasized. When this happens, it is called metastatic or advanced skin cancer. Melanoma is the type of skin cancer most likely to spread to distant organs, most often the lungs, liver, brain, and bones. This can affect how these organs function and may cause additional symptoms. 

Learn more about the signs and symptoms of skin cancer.

Types of skin cancer

There are several types of skin cancer, classified by the type of skin cell where the cancer begins:

The three most common types:

• Basal cell carcinoma (BCC). The most common type of skin cancer, and the most common cancer in the United States overall. It begins in the basal cells of the epidermis and most often appears on sun-exposed areas such as the face, head, and neck. BCC rarely spreads to other parts of the body and is almost always curable when caught early.
• Squamous cell carcinoma (SCC). The second most common type of skin cancer which begins in the squamous cells of the epidermis and can appear on sun-exposed skin as well as inside the mouth, on the lips, or on the genitals. SCC is more likely than BCC to grow into deeper layers of skin or spread to lymph nodes or other organs, though this is still uncommon when caught early.
• Melanoma. The most serious type of skin cancer. It begins in the melanocytes, the cells that give skin its color. Although melanoma accounts for only about 1% of skin cancers, it causes the large majority of skin cancer deaths because of its tendency to spread to other organs if not caught early. Melanoma can develop from an existing mole or appear as a new growth anywhere on the body.

Less common types:

• Merkel cell carcinoma. A rare and aggressive skin cancer that begins in Merkel cells, which are found at the base of the epidermis near nerve endings. It tends to grow and spread quickly.
• Dermatofibrosarcoma protuberans (DFSP). A rare skin cancer that begins in connective tissue cells in the dermis. It grows slowly and rarely spreads to distant organs.
• Sebaceous carcinoma. A rare cancer that originates in the oil glands of the skin, most often appearing on the eyelid.
• Cutaneous lymphoma. A type of non-Hodgkin lymphoma that begins in the white blood cells (lymphocytes) in the skin. The most common form is mycosis fungoides.
• Kaposi sarcoma. A cancer that forms in the lining of blood and lymph vessels in the skin. It is most often seen in people with weakened immune systems.

Related conditions

Certain pre-cancerous skin conditions can increase the risk of developing skin cancer if left untreated:

• Actinic keratosis (AK). A rough, scaly patch on the skin caused by years of sun exposure. Actinic keratoses are considered pre-cancerous because they can develop into squamous cell carcinoma over time.
• Dysplastic nevi (atypical moles). Moles that look different from common moles and may be associated with a higher risk of melanoma.

Skin cancer also belongs to a broader group of cancers that can be associated with inherited gene mutations. Some people with melanoma have hereditary conditions such as familial atypical mole and melanoma (FAMM) syndrome that can increase the risk of melanoma and other cancers.