A familial insertion involving an active nucleolar organiser within chromosome 12.

As far as the authors are aware this is the first report of the insertion of an active NOR into a non-acrocentric chromosome, although a simple translocation involving an active NOR has been previously recorded. More specifically, this case involves the non-reciprocal translocation of the centromere and stalk of an acrocentric into 12p, generating an apparently stable dicentric chromosome. The insertion is seen in three generations and may be relatively genetically benign. The abnormality is fully described by G and sequential C banding, DA/DAPI fluorescence, kinetochore staining, and Ag-NOR staining, and the findings are discussed in the light of the limited published reports of insertion in man.