LONDON -- Scientists who helped to sequence all the genes in the body said Wednesday they have 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 keys to why people are susceptible to diseases such as cancer, diabetes and heart disease and to the best way to diagnose and treat such diseases.

The Human Genome Project mapped the three billion letters that make up the human genetic code. SNPs are single changes within 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," said Eric Lander, the director of the Whitehead Institute Centre for Genome Research.

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

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

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 that are tailored to an individual's genetic profile.

In an accompanying report in Nature, scientists at the Sanger Centre in Cambridge, England, who helped to sequence the first human chromosome, said they 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 that 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.