Moles are colored spots or bumps of various shapes and sizes on your skin. They form when pigmented cells called melanocytes cluster together.

Moles are very common. Most adults have between 10 and 40 of them on various parts of their body. Moles are most likely to form on areas of skin that have been exposed to the sun. You’re more likely to get moles if you’re fair skinned and often in the sun.

The majority of moles are harmless. These are called common moles. They rarely turn into cancer, unless you have more than 50 of them.

Less common are atypical moles (dysplastic nevi). These moles aren’t cancerous, but they can turn into cancer. About 1 out of every 10 Americans has at least one atypical mole. The more of these moles you have, the greater your risk of developing melanoma — the deadliest type of skin cancer. Having 10 or more atypical moles increases your risk 14-fold.

Because an atypical mole has the potential to turn into melanoma, knowing which type you have and watching for any changes can help you get an early diagnosis if it is cancer. Experts suggest that you do monthly skin self-exams, checking your entire body — including less obvious areas like the soles of your feet, your scalp, and the skin underneath your fingernails — for any new or changing growths.

A mole can be a flat spot or large bump. Normal, common moles have these characteristics:

  • They are one color, such as brown, tan, red, pink, blue, clear, or skin-toned.
  • They measure less than 1/4 inch (5 millimeters) across.
  • They’re round and even on both sides.
  • They have a well-defined border that separates them from the rest of your skin.
  • They don’t change.

An atypical mole can form anywhere on your body, including your head, neck, scalp, and torso. They rarely appear on the face.

Atypical moles can also be flat or raised. They also have these characteristics:

  • They measure more than 1/4 inch (5 mm) across — larger than the size of a pencil eraser.
  • They are irregularly shaped, with uneven borders that may fade into the skin around the mole.
  • They contain more than one color, including a mixture of brown, black, tan, pink, and white.
  • Their surface may be smooth, rough, scaly, or bumpy.

Examine your skin once a month in front of a full-length mirror. Check every part of your body, including:

  • your scalp
  • the backs of your arms
  • your palms
  • the soles of your feet
  • in between your fingers and toes
  • the back of your neck
  • behind your ears
  • between your buttocks

If you can’t see all of these areas yourself, ask someone to help you look. Keep a record of any new spots, and check them often to see if they’re changing. If you have atypical moles, you should also see your dermatologist for checkups every six months to one year.

Any new, suspicious-looking, or changing spots should prompt an immediate visit to your dermatologist. Although most atypical moles never turn into cancer, some of them can. If you do have melanoma, you want to have it diagnosed and treated early, before it has a chance to spread.

Your doctor will examine your moles. He or she will probably take a sample of tissue from one or more of the moles. This test is called a biopsy. The sample will go to a laboratory, where a specialist called a pathologist will check to see if it’s cancer.

If your dermatologist finds that you have melanoma, your close family members should probably get checked, too.