A naturally occurring protein that could "revolutionize" cancer treatment produced promising results in the first study testing it on patients with malfunctioning bone marrow, researchers said. Results of the study, headed by Dr. Jordan U. Gutterman, chairman of the department of clinical immunology and biological therapy at the M.D. Anderson Hospital and Tumor Institute in Houston, were published in The New England Journal of Medicine. The substance--granulocyte-macrophage colony-stimulating factor or GM-CSF--sharply boosted the production of healthy blood cells in eight patients with myelodysplastic syndromes, a group of diseases caused by damaged bone marrow. GM-CSF has recently been reproduced using genetic engineering techniques.