The RBM (RNA-binding motif) gene family encodes proteins with an RNA binding motif. RBMY (RBM, Y chromosome) encodes a germ-cell specific nuclear protein involved in spermatogenesis. The RBM gene family, including RBMY1A, RBMY1B, RBMY1D, RBMY1E, RBMY1F, RBMY1H and RBMY1J, is comprised of at least 30 genes and pseudogenes, found on both arms of the Y chromosome. RBM X, an ancestral X chromosome homolog of the RBMY gene, encodes hnRNP G, which is widely expressed, whereas the RBMY gene evolved a male-specific function in spermatogenesis. Micro-deletions of the AZFb region of the Y chromosome, which contains a number of RBMY genes, usually result in severe consequences for spermatogenesis. RBM expression is localized to the nuclei of germ cells and RBM interacts with Tra2beta. Tra2beta is a ubiquitous activator of pre-mRNA splicing, but is most highly expressed in testis, suggesting a role for RBM in Tra2beta-dependent splicing in spermatocytes. The human RBMX gene maps to chromosome Xq26 and the RBMY gene family is found on all mammalian Y chomosomes.