If there was one thing working in their favor, Auerbach told them, it was that their children's disease was diagnosed so early in life. Fanconi anemia is rare, and few doctors have ever seen a case, which means the condition is often missed or mistaken for something else. Auerbach has seen too many children with this same Fanconi mutation whose blood fails, with little prior warning, at age 5. Those parents don't have time to do the only thing there is to do, the one thing the Nashes and Strongin-Goldbergs could do -- have a baby.

Ten weeks into a pregnancy, Auerbach explained, a chorionic villus sampling test can determine whether the fetus is healthy and if it is a compatible donor. Couples regularly abort when they learn that the unborn child has Fanconi, Auerbach says; having seen the devastation wrought by the disease on one of their children, they refuse to allow it to claim another. Few couples abort, however, when they learn that the baby is healthy but not a donor. ''Only three that I know of terminated for that reason,'' she says. ''They were getting older, their child was getting sicker and they were running out of time.'' Far more common, she says, is for couples to keep having children, as many as time will allow, praying that one will be a match.

Timing a child's transplant means playing a stomach-churning game of chicken with leukemia. The younger a patient is when undergoing a transplant, the better the outcome, because the body is stronger and has suffered fewer infections. On the other hand, the longer the transplant can be delayed, the greater the odds of conceiving a sibling donor, and the better the chance that transplant technology will have improved. The risk of waiting is that every Fanconi patient will develop leukemia, and once that happens a transplant is all but impossible. ''You want to wait as long as you can,'' Auerbach says, ''but not so long that it's too late.''

Good doctors learn from their patients, and so it was when Dr. John Wagner answered his telephone one afternoon seven years ago. A lanky, easygoing man, Wagner is scientific director of clinical research in the Marrow Transplant Program at the University of Minnesota, and he says he believes he has performed more bone-marrow transplants on Fanconi children than any other doctor in the country. The caller who set him thinking, however, was not the parent of a Fanconi patient, but rather the father of a toddler with thalassemia, another rare blood disease. The man was calling to inquire about a sibling cord-blood transplant. ''You have another child who is a match?'' Wagner asked. ''No,'' came the reply. ''But we will.''

The father went on to explain that he and his wife were using a relatively new technique known as pre-implantation genetic diagnosis, or P.G.D., to guarantee that their next child would be free of thalassemia. P.G.D. is an outgrowth of in-vitro fertilization; sperm and egg are united in a petri dish, and when the blastocyst (it is still technically too small to be called an embryo) reaches the eight-cell stage, it is biopsied (meaning one of those cells is removed and screened). Only blastocysts found to be healthy are returned to the womb. Then the waiting game begins -- more than two months until it is possible to know if the fetus is a transplant match, then an agonizing choice if it is not. Why, the caller wondered, can't the donor-compatibility tests be done before the embryos are implanted?

Wagner was intrigued by the possibility. Why use P.G.D. just as prevention, he wondered, when it could be used as treatment? Why not, in effect, write a prescription that says ''one healthy baby who is going to be a perfect donor''?

Wagner called Mark Hughes, who pioneered the technique and who was working with this family. Hughes is known as a brilliant researcher, simultaneously passionate and wary, a scientist and physician who chose the field of genetics because it combined the intellectual rigor of the lab with the emotional connection to flesh-and-blood patients. In 1994, at about the time he first spoke to Wagner, Hughes was recruited to work at the National Institutes of Health and also as director of Georgetown University's Institute for Molecular and Human Genetics, where his salary was paid in part by the N.I.H. In other words, much of his research was supported by the government. At that time he was also a member of a federal advisory committee that developed guidelines for the type of single-cell embryo analysis that was central to P.G.D. But no sooner had those guidelines been developed than Congress banned all federal financing of embryo research, and Hughes was forced to continue his research with private funds only.