Medical science is scoring research breakthroughs at an exciting and encouraging pace, and some of the most promising work involves those tiny building blocks of heredity, the genes. Two recent developments suggest the pace of progress.

One deals with cystic fibrosis, the most common fatal genetic ailment in the United States. Most people have a healthy gene that prevents the disease; victims of cystic fibrosis cells do not. But laboratory researchers, by deftly inserting healthy genes, have been able to cure defective cystic fibrosis cells.While it may be some time before this achievement can be repeated successfully in the bodies of actual patients, researchers are delighted with what they have found.

In another achievement, a 4-year-old girl with a rare blood disease may soon begin to get well. Her body lacks the crucial gene that is needed to keep immune cells alive, making her easy prey for disease.

Doctors at the National Institutes of Health decided to use genetic engineering techniques to repair the defect. They drew some of her blood, treated it with a copy of the gene she lacks, then replaced the altered blood cells in her body. Doctors hope that the new cells will produce normal amounts of the enzyme needed to make the girl's immune system function properly.

The girl was the first person in history to receive gene therapy. As with the cystic fibrosis study, results hold great promise. One member of the federal panel that approved treatment for the young patient calls gene therapy "conceptually profound."

In time, many researchers and physicians believe, it may become possible to replace defective or absent genes almost routinely, thereby healing other hereditary disorders.

Gene therapy, seen as radical and even dangerous in its first tentative stages a few years ago, has progressed far faster than most scientists foresaw.

Equally encouraging has been the meticulous review process required before any groundbreaking new genetic treatment is undertaken. Given such careful supervision, the risk of a genetic experiment getting out of hand is neglible.