There is still one disease for which bloodletting is the cure.

It is called hemochromatosis, which results from a genetic defect that prevents the body from controlling the amount of iron it takes in. Toxic levels accumulate, and bloodletting every so often is the life-saving treatment.

Unfortunately, the disease is not easily diagnosed, and many die from liver or heart damage before it is detected.

Because iron is one of the most common elements on Earth, it is not surprising that all living things depend heavily on it to exist. But iron also is poisonous to cells.

How cells manage this poison with precision is an important question to researchers, and, in delving into the mystery, scientists uncovered a new mode of chemical regulation in the cells.

A group headed by Dr. Richard Klausner, cell biologist at the National Institute of Child

Health and Human Development, reported in the Dec. 11 issue of Science the discovery of the bit of genetic material that regulates iron levels in the cell.

It is a so-called genetic switch, made of RNA, that calls for the production of an iron-neutralizing protein called ferritin when too much iron comes into the cell. Ferritin, in turn, builds a tiny chemical cage that sequesters and holds up to 4,500 molecules of iron for future use.

The RNA switch is called the "IRE," for iron-responsive element. But it turns out that the switch may have importance far beyond iron regulation.

It is the first example in human cells of a class of regulators that operates on RNA (ribonucleic acid) instead of DNA (deoxyribonucleic acid).

DNA contains the code, or gene, for operating a cell and its chemicals. Normally, DNA makes an RNA copy of a message, and from the RNA copy the desired chemical is ordered up. Control of the

manufacture of a chemical is made at the DNA level.

With IRE, the process is different. The DNA generates an RNA copy only to have the next step -- the translation from RNA to the desired chemical -- put on hold. The IRE -- the RNA switch -- "stores" the copy of RNA and activates production of ferritin when the iron concentration becomes too high.

Hemochromatosis, Klausner said, is the most common genetic disease. One in 10 people are carriers and between one in 400 and one in 800 have the disease. He said he hopes that current research might yield a way to diagnose the disease as well as provide another tool of gene regulation.