Radioactive fallout from Soviet atomic bomb tests caused genetic mutations in people living near the test site, according to a new scientific report.
Writing in the journal "Science", the researchers say that fallout almost doubled the normal rate of genetic mutation in families living around Semipalatinsk, in Kazakhstan, formerly part of the Soviet Union.
The USSR conducted atmospheric tests at Semipalatinsk in the 1940s and 50s.
The finding provides striking new evidence of the damage which radioactive substances can do to the human genome.
Chernobyl link
The researchers examined 40 families living downwind of the Semipalatinsk site, and found a mutation rate in their DNA substantially above normal.
In older people the mutation rate was nearly doubled, but younger people were affected less.
The researchers say this variation can be put down to the period when atmospheric tests were conducted.
Four tests between 1949 and 1956, they believe, did most of the damage, so people born later were exposed to lower levels of radioactivity.
It is not known whether the mutations led to any health problems, and the researchers say we are unlikely to find out.
The Soviet Union, along with the rest of the world, banned atmospheric tests in 1963, and the scientists say there are so few people still alive who are old enough to have lived through the tests that accurate research will be impossible.
The scientist who led the project, Yuri Dubrova, from Leicester University in Britain, has previously published studies showing the same kind of mutation rate increase in people affected by the nuclear accident at Chernobyl in 1986.
But that research proved somewhat controversial, with other scientists disputing the findings.
Now that he's found the same phenomenon around Semipalatinsk, Dr Dubrova plans to go back to Chernobyl for further investigations.
Maj Hulten, professor of medical genetics at the University of Warwick, who worked on the research, said: "This study does show that there is an effect on genetic mutation.
"It also would mean that in some other forms [radiation] could hit genes which have relevance for development."
She added that the genetic material which has been collected from the families provides a "biobank" which further research can be carried out on in the future.