In Mary V. Relling's office in St. Jude Children's Research Hospital sits a small ceramic statue of St. Jude Thaddeus, the patron saint of impossible causes.

Dr. Relling, the head of the department of pharmaceutical sciences at St. Jude, has a fondness for impossible causes.

Her own is pharmacogenetics, a clinical discipline in which doctors use high-tech genetic testing to custom-make drugs to patients' individual needs.

Though pharmacogenetics is controversial and not yet widely done, Dr. Relling, 46, travels the country advocating its use. At St. Jude, patients with leukemia are now routinely given genetic tests to determine their individual response to a medication. ''We've seen it save lives here,'' she said. ''That's made me a believer.''

When hundreds of patients are given a drug, she continued, ''some will get no benefit, others will have terrible side effects, and still others will get benefits with tolerable side effects.''

Gene variants may be the cause.

Q. How is this tailoring of drugs different from the way they're currently ordered?

A. Till now, there's been a one-size-fits-all approach. In most cases, an average dose of a medication is ordered, and then, if the patient suffers side effects, the dosage is adjusted. With gene testing, we can customize the prescription.

Here at St. Jude, we've been gene-testing every child who comes to us with leukemia. I study acute lymphoblastic leukemia -- A.L.L., the most common childhood cancer. When a youngster comes in with A.L.L., we get a sample of their DNA. We put it on a special computer chip that scans a half-million different places on the genome. Mostly, we're looking for unusual variations of the genes and misspellings of the genetic code.

We have a database from earlier patients that helps us predict a patient's risk of relapse and which misspellings are likely to result in drug sensitivities.

Q. Are there other diseases where the process might be useful?

A. The same medicine we use to treat leukemia is also prescribed for Crohn's disease and ulcerative colitis. So that same genetic test could be employed to reduce side effects with those conditions.

At the moment, there seems to be a lot of promise for pharmacogenetics in the treatment of arthritis, heart disease, colon cancer and even psychiatric diseases like depression and schizophrenia.

Q. One can almost hear economists everywhere groaning, ''Oh, no! Not another test to add to health care costs!''

A. The basic research behind pharmacogenetics -- figuring out which genes are important with the various drugs and diseases -- is costly. But on the clinical level, you can save money. With leukemia, we've seen that testing costs are minor compared to the savings gained by avoiding drug reactions, blood transfusions and additional hospitalizations.

Q. How widespread is genetic testing for prescriptions?

A. It's very rarely used. Most probably, you can find it at some academic centers in big cities. And, of course, for many medications, the research isn't in yet about which genes are important and why. But even where tests have been approved, insurers don't cover all the costs, and that's hindered this from growing.

Most prescribers don't understand genetics very well. The fruits of the Human Genome Project have only been out for about five years, and a lot of doctors and pharmacists did their training before that.

I've heard people say that medicine won't change until there are major lawsuits against prescribers who fail to use tests to individualize therapy.

Q. How have the drug companies responded to the promise of pharmacogenetics?

A. Unfortunately, they are not set up for it. The big pharmaceutical companies have a different business model. They make their money from blockbuster drugs that reach millions of people with standardized doses. They don't want their markets to fragment, which is the obvious effect of pharmacogenetics.

Genomic testing is going to mean that we define smaller and smaller markets for every drug. Instead of one medication for high blood pressure, a manufacturer will have to produce dozens of variants and combinations.

It's already difficult to get drugs that benefit smaller numbers of patients. ''Orphan'' drugs are often not manufactured because they help only a small group.

In pediatric cancer, we see what happens to orphan drugs because children's cancers are rare and, thus, orphan diseases. Of the nine drugs we regularly use for leukemia, seven have been unavailable for varying periods of time during the last decade.

Q. What can you do in that situation?

A. We're trying to start making it here at St. Jude. We're not a drug manufacturer, and it isn't easy for us to do. We're also working with pharmaceutical companies to see if we can't help them make it available. But we can't depend on market forces for our supply.

We've seen that the pharmaceutical companies, if they are interested in cancer drugs at all, are mostly concerned with adult cancers -- the larger market.

Q. How did you become a pharmacist?

A. I attended the University of Arizona in the late 1970's, as a French major. In my freshman year, I took the required chemistry course with a brilliant teacher, William Lippincott. He made chemistry come alive. Becoming a pharmacist seemed like a practical route to a chemistry career, with good job possibilities after graduation.

Even today, it's a great profession for a young person to consider. There is a tremendous demand.