Syntenic chromosomal areas share paralogous genes which are believed to have been generated by repeated duplication of an ancestral gene. The human RING3 gene is known to have paralogous relationships with the ORFX, BRDT, and HUNK1 genes. In addition to the mouse Ring3 cDNA clones previously reported, we isolated mouse Orfx, Brdt, and Hunk1 cDNA clones using mouse testis RNA. Among these four paralogous genes, structure and expression profiles were compared. The proteins encoded by these genes exhibited similar amino acid sequences including two conserved bromodomains. While the Ring3, Orfx, and Hunk1 genes were ubiquitously expressed in various tissues of adult mouse, the Ring3, Orfx, and Brdt genes produced testis-specific transcripts and the Hunk1 gene produced a striated muscle-specific transcript. The diversification of expression patterns of Ring3-related genes during evolution may reflect nucleotide variations in regulatory elements associated with ubiquitous or tissue-specific gene expression.