(MONEY Magazine) – The nightmare began two weeks after Mary Lee Horton, now 34, of Wilmington, N.C. gave birth to identical twin girls, Sydney and Kate. On Feb. 24, 1990, Syd came down with a 103 fever. A blood test revealed that she had extraordinarily low red blood cell and platelet counts. Biopsies at the University of North Carolina Medical Center in Chapel Hill confirmed the worst: Syd was suffering from familial erythrophagocytic lymphohistiocytosis (FEL), a rare and fatal genetic blood disease. Two weeks later, Kate developed similar symptoms. The Chapel Hill doctors grimly advised Mary Lee and her husband Howard Mitchell, 39, that no child with FEL had survived past its fourth birthday. They also told them that the best possible treatment was chemotherapy and steroids. Mary Lee immediately decided to research the disease on her own. Aided by a national network of parents coping with FEL, she wrote to a doctor in France who had successfully treated the disease with bone marrow transplants. He told her that Sloan-Kettering Memorial in New York City was experimenting with his technique. On Dec. 28, Mary Lee mailed samples of Syd's and Kate's blood to Sloan- Kettering so the type could be matched with donors. Mary Lee says that hospital errors caused the hunt for donors to take more than a year, but the hospital denies responsibility for the delay. For Syd, sadly, the wait for a donor match proved fatal. She died of pneumonia on Oct. 13, 1991. But three months later, Kate got the new bone marrow she needed. Today, though she must visit Duke University (in Durham) for a blood test and an immune booster shot three to four times a year, the lively, blonde four-year- old otherwise lives a normal life, taking ballet and gymnastics lessons. Looking back now, Mary Lee says her biggest regret is that she did not press her doctors hard enough, and early enough, for alternatives. "Friends also say we should have sued Sloan-Kettering," says Mary Lee. "But we didn't want money. All we wanted was Syd back."