Source and component genes of a 6-200 Mb gene cluster in the house mouse

We identified and analyzed the genes Sp100, Csprs, and Ifi75 in two members of the genus Mus, M. musculus and M. caroli. Sp100 is a nuclear dot gene; Csprs and Ifi75 are novel genes encoding a putative G-protein coupled recep tor (GPCR) and a putative transcriptional coactivator. respectively. A four th gene. Sp100-rs, occurs in M. musculus, bur not in M. caroli. Sp100-rs is a chimeric gene which arose by fusion of Sp100 and Csprs copies. Sp100-rs and Ifi75 are components of a repeat cluster that extends over 6-200 Mb of the M. musculus genome. The Sp100-rs fusion gene arose only 1-2 million yea rs ago and has become fixed and amplified in M. musculus. Although the gene is transcribed, it appears to have no function. The repeat cluster may hav e become fixed in the species as a 'hitchhiker' in a 'selective sweep'.