A search for genes with sequence homologies to the FMR1 gene resulted in the isolation of mouse and human homologues of the recently described FXR1 gene. The mouse FXR1 gene shares amino acid identity and similarity of 99.1% and 99.6%, respectively, with the human FXR1 gene and amino acid identify and similarity of 67.3% and 79.5% respectively, with the mouse FMR1 gene. The 3' untranslated region of the FXR1 gene is extremely conserved between human and mouse. The gene structure of FXR1 is very similar to that of FMR1 and both genes probably originate from a common ancestral gene. In contrast to the previously published localization, we mapped the transcribed gene to chromosome region 3q28. An intronless form of the FXR1 gene, either processed functional homologue or pseudogene was localized to 12q12. Northern blot analysis of the human FXR1 gene revealed an expression pattern of a housekeeping gene with stronger expression in muscle. RNA in situ hybridization to sections of mouse embryo and adult tissues has shown that during embryonic development the mouse FXR1 mRNA is expressed in different tissues, most prominent in skeletal muscle, the gonads and distinct regions of the central nervous system, and that the expression is restricted to proliferating cells. While FMR1 is highly expressed in proliferating spermatogonia, FXR1 is highly expressed in postmeiotic spermatids.