RBMX and RBMY are members of an ancient pair of genes located on the sex chromosomes that encode RNA-binding proteins involved in splicing. These genes have differentiated and evolved separately on the X and Y Chromosomes. RBMY has acquired a testis-specific function, whereas, as shown here, RBMX is ubiquitously expressed and is subject to X inactivation. We have also found that multiple processed copies of RBMX are present in the human genome. RBMX-like sequences (RBMXLs) located on human Chrs 1, 4, 6, 9 (9p13 and 9p24), 11, 20, and X lack introns and thus probably result from retroposition events. We found RBMXLs to be conserved in primates and great apes at corresponding chromosomal locations, indicating that they arose prior to the divergence of human. Some of the RBMXLs show insertions, deletions, and stop codons, which would probably result in nonfunctional proteins. The RBMXL on Chr 20 is deleted in some individuals. Two of the largely intact RBMXLs, located on Chrs 1 and 9p13, are expressed in different tissues and may encode novel proteins involved in splicing in a tissue-specific manner. The RBMXL located at 9p13 is specifically expressed in testis, and to a lesser extent in brain, and may therefore play a role in testis function. This autosomal, testis-specific copy of RBMX could potentially compensate for RBMX that is presumably inactivated in male germ cells, in a manner analogous to autosomal retroposed copies of other X-linked genes.