Silent ribosomal cistrons are located at the pairing segment of the postreductional sex chromosomes of Apodemus sylvaticus (Rodentia, Muridae).

We analysed the karyotype of the rodent species Apodemus sylvaticus by G- and C-banding, Ag-NOR-staining and in situ hybridization, with special attention to the sex chromosomes. NOR-bearing chromosome pairs were identified with simultaneous staining of G bands and NORs. In situ hybridization with an rDNA probe revealed the presence of silent ribosomal cistrons in both sex chromosomes. Studies of meiosis demonstrated that these inactive ribosomal cistrons are located in the pairing segment, which occupies the proximal portion of the telocentric sex chromosomes, and may thus be involved in postreduction caused by an obligatory chiasma in this position.