Pinpointing faulty genes is the first step to a detailed understanding of certain illnesses and, ultimately, more effective treatments.

When a version of the human blueprint appeared in 1996 it took scientists just weeks to find 16 disease-related genes.

The new map, which will also be published in a forthcoming edition of the journal Science, gives details of about half of our genes and marks the half-way point in the Human Genome project.

"By any standard it is a tremendous step forward," said James Watson of Cold Spring Harbour Laboratory in New York.

Mr Watson, along with Francis Crick, won the Nobel prize for discovering the structure of the DNA molecule in the early 1950s.

More accurate

This international effort aims to map all of the 100,000 or so genes that can be found in the vast majority of cells in our bodies.

Inside almost every human cell there are 23 pairs of chromosomes, each made of a strand of DNA. Sections are grouped together to form genes, which make the chemicals we need to live.

Scientists anticipate that with more accurate techniques they will have a working map of human genes by 2001.

Two years later, when they hope to have completed the process, the researchers will have what some call The Book of Man.