An experimental type of bone marrow transplant appears to have saved 4-year-old Michael Sancilio from a rare form of blood cancer. Eight months ago, the Virginia Beach boy developed chronic myelogenous leukemia, which is frequently fatal within a year of diagnosis.

The most advanced treatment is a bone marrow transplant. For Sancilio, however, a compatible bone marrow donor could not be found. Instead, doctors saved the blood from the umbilical cord of his sister, Christina, who was born May 1. Normally, these cells are discarded with the placenta, but because of Sancilio's illness they were saved.

Umbilical cord blood contains a rich supply of so-called stem cells, the bone marrow cells that generate all other blood cells. The cells from the umbilical cord can be used just like a traditional bone marrow transplant.

Christina's cord blood sufficiently matched Michael's to allow the transplant to take place. In this procedure, physicians first destroy all the blood cells in the patient's body with drugs or radiation. Then doctors gave Sancilio a transfusion of about five teaspoonsful of his sister's stem cells. A month later, the stem cells were rapidly forming a new bone marrow.

Umbilical cord blood transplants only have been performed three other times: once in Cincinnati and twice in France to treat a a rare blood disorder called Fanconi's anemia.

Tests before Sancilio's release from Johns Hopkins Hospital last week showed no sign of the cancer. "Although doctors are reluctant to say Michael is cured," said a statement from Virginia Beach General Hospital, where he will continue to be followed, "Michael appears to have defeated a rare form of leukemia for which there existed no known cure."