Genes have been transplanted for the first time into animals, a feat that may make it possible to insert them into humans to treat genetic diseases or help defeat cancer.

Dr. Martin J. Cline, head of the research team at the University of California at Los Angeles, predicted yesterday that the technique could be applied to humans in "three to five" years to help treat cancer and relieve such crippling and often fatal genetic diseases as sickle cell anemia.

The achievement was reported in the British science journal Nature. A simultaneous UCLA announcement called the work "revolutionary."

"This shows it is possible to put new genes into animal cells and have [them] function when they are reintroduced into living mammals," Dr. Cline said. "The possibilities for that are enormous."

A gene, chemically, is just a length of DNA or deoxyribonucleic acid from the chromosomes or carriers of heredity inside the body's cells. But these chemicals contain all the hereditary characteristics that make every animal what it is.

Cline and associates -- Winston Salser, Karen Mercola, Howard Stang and others -- successfully inserted two different genes into laboratory mice.

One was the gene from the common herpes simplex or cold sore virus. The other was a gene that produces resistance to the commonly used highly toxic anticancer drug, methotrexate.

The genes first had to be separated and cloned -- grown in quantity in identical form -- by some of the methods that have recently made gene isolation and recombination possible.

Most such work has put new genes into bacteria. But Dr. Richard Axel of Columbia University, as well as others, have recently shown that mammalian cells, far more complicated than bacteria, can also absorb new genes.

In treating cancer, one problem is that the anticancer drug may be so toxic and powerful that it kills too many cells, not just cancer cells, and especially blood-forming cells in the bone marrow.

Using Axel's method, the UCLA scientists took bone marrow cells from adult mice and incubated the cells with DNA containing the desired new methotrexate-resistant gene, for example.

Some of the cells took up the new gene. These cells were then reinjected into the mice. And some went straight to the bone marrow.

When methotrexate was given these animals, the cells that took up the new genes were better able to survive it. They produced rapidly and within two months became the dominant blood-cell producing cells.

The hope in human cancer patients, said Cline, is that these new cells would help the body withstand larger and more effective doses of methotrexate or other anticancer drugs whose use is often limited because they kill too many blood-forming cells.

The UCLA animals bearing the new gene maintained a normal blood supply. In other animals given the anticancer drug, the blood and bone marrow cell counts fell perilously.

In treating sickel cell anemia or thalassemia -- diseases in which patients are born with a defective gene that causes production of abnormal, harmful blood cells -- Cline said doctors would insert two genes. One would correct the defect and another would give the patient resistance to a drug to kill off his or her defective genes.

Though "we don't have the techniques yet" to do this in humans -- and no human trials can be expected for at least three years -- "I think it's likely," Cline said, that in time "techniques will be available for inserting genes into many tissues in the body. Then you could conceivably insert the gene for insulin into people with diabetes, or the gene for growth hormone into people who have a growth hormone deficiency."

A biochemist at St. Mary's Hospital in London said in Nature that a crucial problem in humans would be how to control the new gene, which might become too active -- and harmful. Cline thought drug or other methods might handle the problem.

The most important achievement today, he said, is just showing that "the recombinant DNA people were right" in saying that someday genetic defects might be cured by replacing bad genes.