Skin cancer happens when atypical skin cells grow out of control. It tends to occur on skin that gets exposed to sunlight, but it can develop anywhere you have skin, including on mucous membranes.

If you have a blemish on your skin and wonder if it could be skin cancer, there’s a handy system to help you decide if you should contact a doctor. It’s called the ABCD rule for skin cancer.

The acronym stands for “Asymmetrical, Border, Color, Diameter.” There’s also a variation called the ABCDE rule, where the E stands for “Evolving.”

Read on to learn about the ABCDE rule, how it’s used, and signs that it’s time to contact a doctor.

The ABCDE rule tells you what to look for when examining your skin.

The A stands for asymmetrical. One half of a cancerous spot or mole may not match the other if you were to split the mole in half. Noncancerous moles are typically symmetrical.

B is for border. The border of a cancerous spot or mole may be irregular or blurred, or it may be pink or red in color. A typical spot or mole is likely to have well-defined borders.

Next up is color. A typical mole tends to be evenly colored, usually a single shade of brown. A cancerous spot may not be the same color all over.

It can be several shades of the same color or made up of several colors, including tan, brown, or black. They can even include areas of white, red, or blue.

Amelanotic melanomas are harder to detect. They don’t change melanin, so they’re the same color as your skin. They’re often diagnosed late because of this.

The diameter of the spot or mole is also important. It may be a warning sign if it’s larger than 1/4 inch across (6 millimeters), which is about the size of a pencil eraser.

Also note if the spot is evolving. Spots due to melanoma may grow or change color or shape. They may also start to itch or bleed. Benign (noncancerous) spots and moles don’t usually change.

A note about skin color

Brown and black melanomas are harder to spot on dark skin. If your skin is dark, it’s important to examine your skin closely, taking note of these signs of melanoma:

  • a growth or darker patch of skin that’s growing or changing
  • a sore that doesn’t heal
  • a dark line underneath or around a fingernail or toenail

Acral lentiginous melanoma tends to be more aggressive and is the most common type of melanoma in People of Color, according to the Skin Cancer Foundation (SCF).

It’s likely to develop in hard-to-spot places, such as under fingernails and toenails, or on the palms of your hands and soles of your feet. People with darker skin are at a higher risk of developing a melanoma.

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ABCDE rule for skin cancer and concerning skin lesions. Illustration by Alyssa KIefer.

Research shows that regular, yearly skin exams during doctor visits along with regular skin self-examinations can decrease the depth of melanomas at diagnosis.

The most common sign of skin cancer is a change to your skin like an evolving mole, a sore that doesn’t heal, or a new growth.

Here’s what you need to examine your skin:

  • good lighting
  • full-length mirror
  • hand mirror
  • chair

As you examine your skin, remember the ABCDE rule for skin cancer. For every spot you find, think about asymmetry, border, color, diameter, and evolution.

You can also look for the “ugly duckling.” Moles in a group typically look similar, but melanomas generally stand out from the crowd by size, shape, or color.

The American Academy of Dermatology (AAD) suggests these steps to performing a skin self-exam:

  • Examine the front and back of your body in a full-length mirror. Turn and lift your arms to check the sides.
  • Bend your elbows to check your underarms, forearms, and palms.
  • Sit down to check the backs of your legs and feet, the spaces between your toes, and soles.
  • Use the hand mirror to check the back of your neck. Part your hair to check your scalp.
  • Use the hand mirror to check your back and buttocks.
  • Use the hand mirror to check in your groin area.

Noncancerous skin spots that can look like cancer include:

  • Dermatofibromas. Dermatofibromas are small, firm bumps ranging from pink to red to brown in color.
  • Keloids. Keloids are raised growths that develop near wounds and are usually darker than the surrounding skin. They’re more common in people with dark skin.
  • Moles. Moles are dark clusters of pigment-producing cells. They’re usually benign but can become cancerous.
  • Seborrheic keratoses. Seborrheic keratoses are round or oval spots that are tan, brown, or black. They may appear in groups. They also tend to appear as you get older, are usually scaly, and look like they can be picked off.
  • Skin tags. Skin tags are small flaps of skin that match the color of your skin.

According to the SCF, when melanoma is detected before it reaches the lymph nodes, the 5-year survival rate is 99 percent. So any spot or mole that lines up with the ABCDE rule should be examined by a doctor.

Other warning signs include:

  • a spot that differs from nearby spots
  • a spot that itches, bleeds, or causes pain
  • a sore that won’t heal

When in doubt, it’s best to make an appointment to have it checked out. In the meantime, take a photo of the spot so the doctor can see if it’s evolving.

There are several types of skin cancer, with the most common being:

  • basal cell carcinoma, which starts in round cells under the squamous cells
  • squamous cell carcinoma, which starts in thin, flat cells on the top layer of the epidermis (skin)
  • melanoma, which starts in the lower part of the epidermis in cells that make melanin

Other types include:

  • merkel cell carcinoma
  • kaposi sarcoma
  • cutaneous lymphoma
  • sebaceous gland carcinoma
  • dermatofibroma sarcoma protuberans

According to the Centers for Disease Control and Prevention (CDC), the two most common types are basal cell and squamous cell carcinomas, and both can usually be cured.

Melanoma is less common but more aggressive and causes more deaths. It can also be cured if caught early.

Non-melanoma skin cancers tend to develop on skin that gets a lot of sun exposure. They can vary in color from pearly white to pink to red, though they can have a small amount of brown or black. The surface may be shiny, rough, or crusty.

Melanoma is typically dark brown or black, with variations in color. It can also present as white, blue, and pink. In people older than age 40, it can appear as a dark mole.

The ABCDE rule for skin cancer is a handy acronym that can help you identify potential skin cancers. The letters stand for “Asymmetrical, Border, Color, Diameter, Evolving.”

A new or changing spot or mole on your skin may be a sign of cancer. When in doubt, it’s best to have a doctor check it out.