Partial translocation of NOR and its activity in a balanced carrier and in her cri-du-chat fetus.

Cytogenetic analysis of a balanced reciprocal translocation t(5;13) carrier and her unbalanced 5p- conceptus was carried out. This carrier mother had previously given birth to a child with cri-du-chat syndrome. Silver staining demonstrated the breakpoint on 13p within the nucleolus organizer region (NOR). In the carrier, the NORs both at the original site (13p) and at the translocated site (5p) were silver-stained, indicating that the rRNA genes at both sites were transcribed. The NOR at the derivative chromosome 5 was also silver-stained in the fetus.