The Joint Genome Institute, a federal genome sequencing center in Walnut Creek, Calif., has ordered one of 454's $500,000 sequencing machines but has not yet installed it. Paul Richardson, the institute's head of technology development, said the new approach "looks very, very promising" and could reduce sequencing costs fourfold.

The machine's limitation is that at present it can only read DNA fragments 100 units or so in length, compared with the 800-unit read length now attained by the Sanger-based machines. The shorter read length makes it harder to reassemble all the fragments into a complete genome, Dr. Richardson said, so although microbial genomes can be assembled with the new method, mammalian genomes may be beyond its reach at present.

Dr. Fraser, director of the Institute for Genomic Research in Rockville, Md., also said that the new machine's short read lengths "limit its overall utility at this point."

Jonathan Rothberg, board chairman of 454 Life Sciences, said the company was already able to decode DNA 400 units at a time in test machines. It was working toward sequencing a human genome for $100,000, and if costs could be further reduced to $20,000 the sequencing of individual genomes would be medically worthwhile, Dr. Rothberg said.

There would be little advantage at present in sequencing a patient's entire genome, but in the medicine of the future, complete documentation about an individual's genetic makeup could well provide prognosis or indicate a preferred treatment.

The new technology avoids a pitfall of the Sanger method, which is that the fragments of DNA to be analyzed are first amplified by being cloned in bacteria. But the bacteria cannot handle certain fragments, leaving gaps in the genome sequence. In the new technique, each fragment of DNA is captured in an individual drop of liquid and amplified to 10 million copies with a well-established chemical method known as the polymerase chain reaction.

The 10 million copies from each droplet are then attached to an ultra-small bead, and the beads are dropped into a credit-card-sized grid of 1.6 million wells, where the pyrosequencing takes place. Each time the correct base is added to the fragments of DNA on a specific bead, a flash of 10,000 photons is picked up through the bottom of the wells by the light-detecting chip that sits under the small grid of wells. A computer can reconstruct the sequence of bases composing the fragments stuck to each bead, and from the overlaps between fragments can reassemble the entire genome from which they were derived.