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Nucleolus organizer regions (NORs) of rat chromosomes were stained by the Ag-AS method. The Ag-NORs were found on chromosomes 3, 11 and 12 in the ACI, Wistar Brown and Wistar Lewis inbred strains of rat. The size of the Ag-NOR on each pair of chromosomes varied from strain to strain. Rat-human somatic hybrid cells that retained human and lost some of the rat chromosomes had no Ag-NOR on rat chromosomes 3, 11 or 12. Since NORs can be Ag-stained only if their 18 + 28S rRNA genes are active, the activity of the rat rRNA genes must have been suppressed in the hybrid cells.

Cells from four different mouse-human somatic cell hybrids were stained with quinacrine to identify each metaphase chromosome and with ammoniacal silver by the Ag-AS method to locate nucleolus organizer regions. Each of the hybrids contained human acrocentric chromosomes. None of these human acrocentric chromosomes was stained with silver in any hybrid cell. Diploid cells were available from the human parent of one of the hybrids. In these cells both copies of nos. 13 and 15 stained with silver; the same chromosomes in the hybrid cell were not stained. These results support earlier reports that the expression of human ribosomal RNA (rRNA) genes is suppressed in mouse-human hybrid cells. Further, they suggest that silver staining by the Ag-AS method reflects activity of rRNA genes rather than just the presence of these genes.