We have identified and characterized a novel human gene (Nomenclature Committee of the Genome Database GDB-assigned symbol CXorf1) that maps to the long arm of the X chromosome in Xq27 between loci DXS369 and DXS181, approximately 2.5 Mb centromeric to the FMR1 gene. The CXorf1 gene is conserved in primates, cow, and horse but not in mouse and rat. Northern blot analysis revealed two transcripts, present in the brain and in the G361 melanoma cell line. In situ hybridization experiments performed on sections of human hippocampus showed a clear, uneven localization of the CXorf1 mRNA in specific subfields of this brain area. In particular, CXorf1 was localized in the granular-cell layer of the dentate gyrus and in the CA2-CA3 subfields of Ammon's horn. CXorf1 is one of the first genes from this region to be characterized in detail and, on the basis of its chromosomal location and expression pattern, may have an important function in the brain.