Activity of nucleolus organizer regions (NORs) in embryonic livers from twin minks]

Ag-NOR patterns were studied in hepatocytes from nine mink embryo siblings, including a pair of monochorionic (presumably monozygotic, MZ) twins. Both the number and the size of Ag-NORs per cell were found to be identical in MZ twins. All the other sibs had the patterns different from each other and from the MZ ones. The conclusion is that the NORs activity is a strongly inherited character and the Ag-NOR pattern can be used as a reliable genetic marker to distinguish the twin zygosity.