FRAGILE-X SYNDROME WITH EXTRA MICROCHROMOSOME

A mentally retarded male with Martin-Bell syndrome, who has an extra m icrochromosome and is fta X negative in cytogenetic study is reported. Because of its small size, the origin of the microchromosome could no t be determined. Two other affected males in this family (a cousin and a nephew of the proband) were fragile X positive, 24% and 26%, respec tively. Cytogenetic studies and DNA analysis with the probe St B 12.3 were performed on several members of the family. The proband and the t wo other affected males showed a similar full mutation on the molecula r study. This study emphasizes the importance of molecular analysis in the diagnosis of fragile X syndrome, particularly when cytogenetic st udies demonstrate fra X negative in individuals in families likely to have X-linked mental retardation.