By Corinne Podger BBC science correspondent

DNA tests on a body exhumed near the Pakistani city of Karachi may show it belonged to the murdered American journalist Daniel Pearl.

But local police say the tests could take days, if not weeks, to yield conclusive results.

The tests are looking for specific variations in the gene sequence

DNA is one long sequence of biochemical "code" and contains many thousands of genes. Our genes are located at intermittent points along the DNA, which is neatly packaged into structures called chromosomes.

While we all share some gene sequences - for example, the sequences that tell our bodies to develop feet, instead of hooves or fins - other sequences vary, and it is these variations that make each of us unique.

The aim of a DNA test is to find these variations, which have specific "locations" on our chromosomes.

Contamination problem

Typically, more than 10 locations need to be tested before a laboratory can be sure that it has positively identified a specific person.

The most common test works by taking DNA from biological evidence and making millions of copies of it - enough to allow a laboratory to match it against corresponding samples such as a lock of hair or a relative's DNA.

The tests themselves are time-consuming and they are also extremely sensitive to contamination.

Any organic contaminants from a crime scene, particularly one which is old or has been exposed to the weather, may also be copied by accident, making the results useless.

The combination of these factors, and the need to be absolutely sure of the findings if they are to play a role in a murder enquiry, means that it can take several weeks to get a clear result.