For some kidney dialysis and cancer patients, the drug is a potential lifesaver. By stimulating red blood cell production, it can head off potentially fatal anaemia. For drug makers, Epogen is an extraordinarily lucrative substance -- already its market is worth billions of dollars and it could be worth billions more.

In a patent infringement trial starting today in Boston federal court, arguments will center on who has the right to produce it. Wall Street and the biotech industry will be paying close attention.

Amgen holds patents on the protein, which is known generically as erytrhopoietin and accounted for $1.8 billion in sales last year for the Thousand Oaks, Calif.-based biotech giant. But a smaller company, Cambridge-based Transkaryotic Therapies Inc., or TKT, has found another way to make the protein that makes the drug work. Amgen says TKT is violating the patents it holds on Epogen, but TKT says its process is entirely new and could not have been envisioned when Amgen patented its technique.

While Amgen puts cloned human genes into hamster ovary cells to produce Epogen, TKT induces human cells to make the protein in a laboratory culture. Its version of the drug is now in clinical trials.

Wall Street is interested not just because of the billions at stake over Epogen, but because the case could expose Amgen and other companies holding patents on other valuable proteins to competition from upstarts who find new ways to produce them.

A pretrial ruling last month narrowed the focus of this case and probably lessened the chances it will send shock waves through the industry.

In April, US District Judge William Young ruled that TKT infringed on one of Amgen's patents, limiting the scope of the trial to whether Amgen's patents are enforceable, or whether they were valid in the first place. Experts are divided on whether the ruling puts TKT at a significant disadvantage. The company says it already conceded the patents were infringed but believes they are invalid and unenforceable.

But the issue that most interests the industry is whether Amgen owns the protein or merely one of several processes to make it.

``The key issue for relevance to other cases will be whether a patent exists for the protein itself, which ultimately becomes the pharmaceutical product,'' said Dennis Harp of Deutsche Banc Alex Brown.

A victory for TKT could allow it to produce and sell other lucrative substances that aren't patented at the protein level, such as insulin, human growth hormone and Neupogen -- an immune system boosting drug also made by Amgen. All could conceivably be copied if TKT's technology is validated.

``There's potentially billions of dollars at stake,'' said Rebecca Eisenberg, a University of Michigan law professor.

Transkaryotic -- which has no other drugs on the market -- has more to lose in the case. While it has other potentially lucrative products in its pipeline, it may depend on the technology in question to make those products.

The patent suit probably won't affect attempts by Transkaryotic and its partner, the European drug giant Aventis, to break into the European and Japanese markets for Epogen, worth perhaps $1 billion each.

Amgen, valued at $64 billion, has a stronger product line to fall back on if it loses. Schmidt estimated a 5 percent downward revenue swing if it loses. Amgen shares finished the week at $62.37 1/2, also on the Nasdaq.

Johnson and Johnson is also paying close attention. It holds the domestic rights to sell Epogen for cancer-related uses, and for all uses in several foreign countries. Combined worldwide sales of the drug for all uses totaled about $4 billion last year. ``There will certainly be a lot of observers from Wall Street trying to see which way the judge is winking every day, which way his body language suggests he's leaning,'' Schmidt said.