FACTBOX-The growth of DNA profiling

(Reuters) - DNA profiling, or fingerprinting, is an increasingly important tool in solving crimes, as well as for checking family relationships in immigration and paternity disputes.

Here are five main facts about the technology:

* DNA profiling was discovered in 1984 by geneticist Alec Jeffreys of Britain's Leicester University, who first used it three years later to help solve the murders of Leicestershire schoolgirls Dawn Ashworth and Lynda Mann.

* The technology makes use of the fact that small sections of DNA repeat themselves over and over in a way that is unique to each individual. The length of repeats can be measured at different locations to build up an individual's profile.

* Modern genetic tests typically look at 20 "marker" sections of DNA -- 10 from the maternal and 10 from the paternal line. The chance of two strangers matching on all 20 is less than one in 1 billion. The chance of a match with a relative, though, is much higher and identical twins have identical profiles.

* Scientists collect DNA direct from suspects, using a mouth swab, or from the scene of a crime, by gathering traces of blood, semen, hair or saliva. Crime scene samples are more difficult to analyze, since they may be very small and can contain the DNA of several people.

* Governments around the world are building up DNA databases to match suspects with evidence. The United States has the largest database, with over 5 million profiles. But Britain has the highest proportion of people catalogued, with its 4 million records equal to more than 6 percent of the population.