Unlike the bombs, Chernobyl released a great deal of the radioactive isotope cesium 137. The body cannot distinguish cesium from potassium, an essential nutrient, and so the cesium is taken up by cells and becomes an internal source of radiation.

In addition, the reactor at Chernobyl spewed out an array of non-radioactive chemical pollutants, including highly toxic heavy metals.

So far, the major health problem blamed on the meltdown has been an increase in thyroid cancer in children around Chernobyl, due to emissions of radioactive iodine.

In addition, many residents show chromosome damage, which in some cases can lead to leukemia or other cancers, though not enough time has passed for those diseases to develop. Increases in birth defects and miscarriage have also been reported, but some researchers dispute those findings.

Anxiety in the region runs so high that many researchers say they think it is causing more illness than the radiation is.

The families that were studied in Belarus were healthy, and researchers do not know what the newly discovered mutations mean. The uncertainty is based on the type of DNA that was examined.

The researchers chose regions called minisatellites, which are prone to mutation and likely to show changes when exposed to an agent capable of damaging genes. Mini satellites are good screening tools for mutation, but they are not functioning genes and their purpose is unknown. Mutations in them may not be important.