Melanoma, the skin cancer that killed Maureen Reagan, is expected to strike 51,400 Americans this year and kill 7,800.

It's dangerous because it can spread quickly into the body from the skin. But when melanoma is caught in its earliest stages and removed, it is highly curable.

The main preventable cause of melanoma is excessive sun exposure. So doctors advise avoiding or limiting sun exposure from 10 a.m. to 4 p.m., and using protective clothing and sunscreen with an SPF rating of at least 15. Severe sunburns in childhood may particularly increase the risk of melanoma in later life, so children in particular should be protected, doctors say.

Doctors have long advised people to examine their skin thoroughly and regularly to find melanomas and other skin cancers at early stages. A mole that changes in appearance or size or a new mole is worth checking out with a doctor, physicians say.

Melanomas can develop from or near an existing mole, or away from such blemishes. They appear most often on the upper backs of men or women or the legs of women, but they can occur anywhere on the body.

The warning signs of melanoma are often called the ABCD's:

A — asymmetry. One half of a mole or pigmented spot doesn't match the other half.

B — border irregularity. The edges are notched, ragged or blurred.