Chromosomes with active nucleolus organizer regions (NOR's) were identified by combined Q-banding (in some cases), and silver staining in mouse cell lines. NOR-bearing chromosomes were overrepresented among the chromosomes involved in Robertsonian translocations in LM(TK-), A9, and RAG cell lines. Usually only one NOR-bearing chromosome was seen in any biarmed chromosome; relatively few contained two NOR-bearing chromosomes. Thus the nucleolus plays an important role, but nucleolar fusion is relatively unimportant, in the origin of Robertsonian translocations in the mouse.
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Miller et al. (2008) studied this question.