Spontaneous expression of the chromosome fragile site fra(10)(q25).

We report the spontaneous expression of the chromosome fragile site at 10q25 in a child with progressive cerebellar ataxia and in her phenotypically normal brother and father. Expression of this fragile site was increased in all three individuals by addition of bromodeoxyuridine (BrdU) to the medium. This fragile site has previously been described only following growth of cells in BrdU.