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'.