In conceiving Marissa, the Ayalas were seeking to escape from a desperate predicament. Their daughter Anissa has chronic myelogenous leukemia, a disease that kills 80 to 90 percent of patients within five years of diagnosis. Her only hope is a bone marrow transplant, Dr. Forman said, and even then her survival is far from assured. Twenty to 25 percent of marrow transplant patients die, usually of infections, adverse reactions or a return of the leukemia. Search for a Donor

When Anissa was found to have the disease four years ago, she and her family began searching for someone whose tissue type was compatible with hers and who would be willing to donate marrow. There is discomfort for the donor, who is anesthetized while doctors poke long needles into hip bones and withdraw precious tubes of the dark red marrow.

The patient, meanwhile, has undergone four days of intensive, whole body irradiation followed by high doses of chemotherapy, a process that destroys every cell of his or her own cancerous marrow. As soon as the donor's marrow is drawn, it is dripped into the patient's bloodstream, where it finds its way into the bones and grows there.

But the Ayalas could not find a compatible donor. Neither parent had the right tissue type, nor did their son, Airon, who is 20. A nationwide search for an unrelated donor found none.

The Ayalas, who live in Walnut, Calif., announced last February their decision to conceive a baby as the best hope of finding compatible marrow for Anissa. Abe Ayala, the father, had a vasectomy reversed. The mother, Mary Ayala, was 42 when she conceived.

Dr. Rudolph Brutoco of Covina, Calif., the baby's pediatrician, said that Mrs. Ayala had amniocentesis when she was six months' pregnant and had the fetus's tissues typed. The reason, Dr. Brutoco said, was not to have an abortion -- it was too late by then, Dr. Brutoco said -- but to learn if the baby could be a donor. If the fetal tissues matched Anissa's, doctors would save the baby's umbilical cord blood to give along with her marrow when Anissa had her transplant. Marissa was born on April 3, 1990.

Dr. Forman said he and the other doctors on the transplant team took "an educated guess," that the best time to attempt the transplant would be when Marissa was 14 months old, balancing Anissa's limited time when she would be healthy enough to have a marrow transplant with their desire to allow Marissa time to grow and develop. Anissa was admitted to the hospital on May 22 and her marrow has now been destroyed: she cannot make any red or white blood cells on her own and will die without her sister's marrow.