Almost everyone has moles, a type of skin growth that's usually harmless but in rare cases can become cancerous.

Known medically as nevi, they form when melanocytes, a cell type found mainly in the skin, grow in clusters. Melanocytes produce the natural pigment melanin, the substance that darkens your skin after exposure to the sun.

"A mole can be seen as an unsuccessful attempt by the skin to protect itself from light rays," says Dr. Christoffer Gebhardt, deputy director of the Department of Dermatology and Venereology at Hamburg-Eppendorf Medical Center in Germany.

As the most common type of benign skin growth, moles come in various colors, shapes and sizes Typically brown, they can also be tan, black, red, blue or pink. Most are oval or round and less than 6 millimeters in diameter. They can be smooth, wrinkled, flat or raised, and may have hair growing from them. Moles can be either congenital or acquired. The exact cause of the former is unclear. Acquired moles, which generally appear in childhood or adolescence, are caused by a combination of genetic predisposition and exposure to ultraviolet (UV) radiation from the sun.

So-called age spots, sometimes called liver spots, are also caused by prolonged exposure to UV radiation. They're harmless.

Since a mole can become cancerous, you should be examined by a dermatologist annually if you have a mole that looks unusual, grows or changes.

"Statistically, only one in 10,000 moles develop into malignant melanoma – skin cancer, in other words," says Gebhardt, adding that the more moles you have, the greater your cancer risk.

"If you have more than 50 moles, you're five times more likely to develop skin cancer, and you're 10 times more likely if you have more than 100," Gebhardt says. "You're also at significantly higher risk if a first-degree relative – a parent or sibling – has developed skin cancer."

It can be difficult to decide whether a skin growth looks suspicious or not. But as dermatologist Dr. Katharina Schuerings points out, "The ABCDE guide is an important self-examination tool that can help you assess moles."

A is for asymmetrical shape. B is for a border that's irregular. C is for a color that has changed or is uneven; the more colors a mole has, the more suspicious it is. D is for a diameter of more than 6 millimeters. E is for evolving – a mole that has changed in size, shape, color or height, or starts to itch or bleed.

If you have a mole that seems suspicious or any skin growth that you're uncertain of, you should see a dermatologist. When detected and treated early, skin cancer can usually be cured. A cancerous mole will need to be surgically removed.

Sometimes moles are removed for aesthetic reasons or because they're located in places where they cause irritation, for example under a bra or in the area of the waistband. Schuerings notes, however, that scars on the torso – compared with those on the face – don't heal very well.