Scientists who helped to sequence all the genes in the body said on Wednesday they had devised a method to produce a more detailed map of the million or more variations in human DNA that make people unique.

The variations are called SNPs, single nucleotide polymorphisms. They hold the key to why people are susceptible to diseases such as cancer, diabetes and heart disease and the best way to diagnose and treat them.

The Human Genome Project mapped the three billion letters that make up the human genetic code, or book of life. SNPs are single changes in the arrangement of those letters that make people different.

``A high-resolution SNP map that is accurate, reliable, and freely available to medical researchers worldwide will greatly accelerate the discovery of the genetic underpinnings of the most common diseases that affect human populations,'' Eric Lander, the director of the Whitehead Institute Center for Genome Research, said in a statement.

Any two humans are 99.9 percent similar. It's the 0.1 percent difference in DNA that accounts for an individual's uniqueness that researchers working for the SNP Consortium, a collaboration of scientists at academic institutions, drug companies and private foundations, are now charting.

Lander and his team have developed a sequencing method, called reduced representation shotgun sequencing (RRS) that increases the accuracy of SNP mapping. Their research is published in the science journal Nature.

COMPARING DIFFERENCES TO TRACK DISEASES

The method looks at specific subsets of the genome from several people and compares the resulting sequences to identify specific DNA variations that cause susceptibility to disease.

``The human genome sequence is the highest resolution map of the structure of our genes -- the DNA letters that make them up. What we are now building is a map of the variation in the DNA,'' David Altshuler, the first author of the Nature paper, told Reuters.

``The variation map is very useful for tracking individual genes or whole regions or chromosomes...tracking different copies in different families or populations so that you can ask if inheritance of any given gene tracks with any disease.''

The ultimate goal of SNP technology is the development of customized medicine and prevention strategies tailored to an individual's genetic profile.

In an accompanying report in Nature, scientists at the Sanger Center in Cambridge, England who helped to sequence the first human chromosome said they had found 2,730 SNPS on chromosome 22. It is the second smallest human chromosome and has genetic components linked to 35 diseases and syndromes including some cancers, schizophrenia, deafness and heart disease.

Each chromosome is made up of a molecule of DNA in the shape of a double helix which is composed of four chemical bases represented by the letters A (adenine), T (thymine), G (guanine) and C (cytosine). The arrangement or sequences of the letters determines the cell's genetic code.

The two papers in Nature are the first published reports of SNP methodology and the progress of the consortium.

The total number of SNPs now available in the public domain is more than 1.2 million, the researchers said.

(From China Daily)