If you see any skin changes at all that worry you, you should see a doctor who may refer you to a dermatologist who can recognise a mole that could cause trouble. With the help of mirrors or a friend, you can make a basic check yourself, paying particular attention to any new moles that might appear after the teenage years and any that change in appearance. The National Cancer Institute in the United States, where melanoma is more common than in the UK, offers this guide as to what to look for:

* Asymmetry. One half of the mole does not match the other.

* Border. The edges are often ragged, notched, blurred, or irregular in outline; the pigment may spread into the surrounding skin.

* Colour. The colour is uneven. Shades of black, brown, and tan may be present. Areas of white, grey, red, pink, or blue may also be seen.

* Diameter. There is a change in size, usually an increase. Melanomas are usually larger than the eraser of a pencil (five mm or 1/4 inch).

It is not simple, however. Some melanomas show only one of these features while some show several. Sometimes the first sign may be that an existing mole develops blacker patches - only a small change, but if detected, the cancer can quickly be stopped in its tracks. Sometimes the melanoma may be a new, blackish and ugly-looking mole. Those who need to check their moles most carefully are those with fair skin and fair or red hair who are likely to burn in the sun rather than go brown. Those with a large number of moles are also at higher risk. Freckles - light-brown, even-coloured and fairly regular patches - are not a problem. They tend to increase in number after exposure to the sun and can also fade away. Others at risk are those from families where more than one member has had a melanoma. Anyone who has had melanoma once has a high risk of getting it again.