Scientists will today unveil the first draft of the human genome, a colossal map detailing our genetic make-up. It is already being called the "Book of Life".
In time a fuller understanding of the human genome will lead to new drugs, advanced treatment of genetically-related diseases such as cancer and heart illnesses, and hold the prospect of extending human life expectancy by several decades. It should also explain why some people are tall or short or have blonde or red hair.
The announcement in Washington will mark the culmination of a decade of research into identifying tens of thousands of human genes by "sequencing" their 3.1 billion DNA sub-units. These are codes for making proteins which can determine how an organism looks, metabolises food, fights infection and sometimes how it behaves.
But it may take the rest of the century to work out fully the genetic coding that makes everyone a distinct individual while having 99 per cent the same genes - each cell has a copy of the same DNA organised into 23 pairs of chromosomes while the totality of the DNA is the genome.
It is still not known how many human genes there are. It could be 80,000 or more.
Two competing project teams will make a joint announcement; the publicly funded Human Genome Project (HGP) which has international backing and Celera Genomics, a private US company in Maryland, which accelerated work on the genome with a revolutionary new method for "mapping genes" at high speed.
Until late last week the American scientists were still trying to strike an accord with Craig Venter, head of Celera Genomics, which entered the field late but threatened to finish first - and offer the human genome for sale.
This move provoked furious exchanges between human genome scientists and Dr Venter. The scientists have always insisted that the data should be available freely, immediately, and to everybody.
HGP has used sperm and blood samples from a number of individuals to produce its version of the human genetic code, while Celera has apparently used samples from one individual.
All human beings have the same genes but there are very slight differences in the DNA sequencing. This is why DNA samples can already be used in criminal investigations to exclude suspects.
DNA testing was used in a recent experiment to show that 98 per cent of men living in parts of Connacht are descended from migrants who arrived 4,000 years ago. Maybe we should not get too excited about today's announcement.
The president of the California Institute of Technology and 1975 Nobel prizewinner for medicine, Dr David Baltimore, said:
"It confirms something obvious and expected, yet controversial: our genes look much like those of fruit, flies, worms and even plants . . . the genome shows that we are all descended from the same humble beginnings and that the connections are written in our genes."