DOCTORS FIND 3RD `FRAGILE SITE' FOR RETARDATION

British doctors said Thursday they had found a third form of mental retardation caused by "fragile site" damage to chromosomes.

"Fragile-X" syndrome is the most common cause of mental retardation after Down syndrome, and is caused by damage to the X chromosome. Chromosomes carry the genes, which in turn carry the genetic codes for living organisms.Another site, much rarer and known as FRAXE, is linked with a second form of mental retardation.

Christopher Jones of Cambridge University and colleagues around the world said they found a third site, associated with a rare form of mental retardation known as Jacobsen syndrome.

They called the site FRA11B, and said they were able to damage the site in laboratory tests.

"It is clear that inheritance of FRA11B does not necessarily lead to chromosome deletion and consequent Jacobsen syndrome," they wrote in the journal Nature.

More research was needed, they said, to find out how the chromosomes get damaged.