Individuals with abnormal phenotype and normal G-banding karyotype: improvement and limitations in the diagnosis by the use of 24-colour FISH

The simultaneous identification, by fluorescence in situ hybridisation (FIS H), of each chromosome in a distinct colour became feasible a few years ago . The key question in the application of this and many other developments i n molecular cytogenetics to clinical situations is whether the results add significant further information that is relevant to the diagnosis. So far, limited data exist regarding how much improvement the technique brings to t he diagnosis of phenotypically abnormal individuals in whom no abnormalitie s have been detected by conventional G-banding analysis. Because of the lac k of a conclusive diagnosis, genetic counselling, estimation of recurrence risk and prenatal diagnosis of these individuals and their relatives is pro blematic. We report a study with 24-colour whole-chromosome painting of 10 familial and Il isolated cases with abnormal phenotypes and normal G-bandin g karyotypes. Previously undetected unbalanced translocations were revealed in two cases. The value and current cost-effectiveness of multicolour FISH for cytogenetic diagnosis is discussed.