Breakpoint within the nucleolus organizer region resulting in a reciprocal translocation t (4;14)(q21;p12).

Reciprocal translocations involving a break in the nucleolus organizer region (NOR) are rare. A balanced translocation in a mother and her fetus with breakpoints in the NOR at 14p12 and on the long arm of a chromosome 4 at band 4q21 is described. The rearrangement was characterized by Ag-NOR staining, multiplex fluorescence in situ hybridization (M-FISH), and FISH with rDNA probes. This and other cases with breakpoints within NORs are discussed.