U3 small nucleolar RNA, which is believed to play a role in eukaryotic rRNA processing, is encoded by a small family of genes (5–10 copies/haploid genome) in mammals. In mouse, functional genes encoding the major U3B RNA form have been isolated, with all copies identified so far having evolved in a tightly concerted fashion. However, knowledge of the precise number and relative localization of all gene-family members has been hampered by the presence of multiple copies of U3B-processed pseudogenes in the mouse genome. In this study, we took advantage of a probe that is specific for functional U3B genes to address this question, using both Southern hybridization of genomic DNA and in situ hybridization of metaphase chromosomes. We show that the mouse haploid genome contains four functional U3B genes, all of which are clustered in a single chromosomal locus. They map to the C-D bands of mouse chromosome 11, within one of the most extended segments of gene-linkage conservation known between the mouse and human genomes, corresponding to a major portion of human chromosome 17. By contrast, the multiple (nonfunctional) U3 retrogenes are dispersed over several mouse chromosome
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