A major project will attempt to map out just how genes are controlled in the human body, scientists in the UK and Germany have told BBC News Online.
Scientists finished decoding human DNA this year
The Human Epigenome Project will look for patterns in our "life code" that are associated with gene regulation but are also implicated in causing disease.
Researchers at Epigenomics AG in Berlin and the Sanger Institute in Cambridge will take part in the five-year study.
It was the Sanger centre which decoded one-third of the DNA found in humans.
Along with their German colleagues, the UK institute staff will now search through the full sequence for sites of so-called methylation.
Levels of complexity
These are locations along the DNA molecule where the structure of one of its four constituent units - the base cytosine - has been modified by the addition of a methyl chemical group.
Changes to the DNA sequence - the order of the bases cytosine (C), guanine (G), adenine (A) and thymine (T) - and its role in health and disease are increasingly well understood thanks to the completion of the Human Genome Project.
THE DNA MOLECULE
The double-stranded DNA molecule is held together by chemical components called bases
Adenine (A) bonds with thymine (T); cytosine(C) bonds with guanine (G)
Groupings of these letters form the "code of life"; there are about 2.9bn base pairs in the human genome wound into 24 distinct bundles, or chromosomes
Written in the DNA are 30,000 genes which human cells use as templates to make proteins; these sophisticated molecules build and maintain our bodies
But DNA methylation is a further layer of control and is regarded by scientists as one of the most important regulators of gene activity.
This has led some to dub the methyl-tagged cytosines "the fifth base".
Their precise pattern is thought to determine whether or not certain genes are expressed in particular tissues.
And as well as being important for normal development, methylation changes are detected in many cancers and some developmental disorders such as Beckwith-Wiedemann syndrome.
Methylation is linked to ageing, too, as the patterns change over time.
It is also thought this crucial system is influenced by a person's upbringing and lifestyle. In effect, it is one of the means by which our environment influences our genetic make-up.
"The fifth base is like a switch - switching on the genes and making cells healthy or diseased," Alex Olek, the chief executive of Epigenomics, told the BBC.
"One of the first things that will come out of this will be very powerful early tools for the detection of cancers. Based on this very mechanism we have a product in the pipeline for the early detection of bowel cancer. This should be on the market in 2006-7."
The German company has been working on an EU-funded pilot, identifying 100,000 methylation sites on a segment of the DNA bundle known as chromosome six.
The full-scale Epigenomics/Sanger work will now extend the pilot to take in the rest of the human genome.
The data obtained in the multi-million-pound project will be released in specific batches following its discovery and made available on the internet.
The Human Genome Project, which set out to read all 2.9 billion bases in human DNA, was completed earlier this year.
The finish coincided with the 50th anniversary of the discovery of the structure of DNA by scientists at Cambridge University and Kings College London.
Researchers believe about 30,000 genes are written in the code.
They will spend many years trying to work out precisely what the genes all do and how they interact to build and maintain the body's cells.