Since the Seventh Fragile X and XLMR Mental Retardation (XLMR) Workshop in 1995, the genes for Coffin-Lowry, Mohr-Tranebjaerg, and Opitz G/BBB syndromes have been cloned. Jensen syndrome has been found to be allelic to Mohr-Tranebjaerg. Twenty new XLMR syndromes and metabolic or neuromuscular disorders have been reported. Twenty-four new localizations have been established, including five in previously reported conditions (FG, Carpenter, Arts, OPA2, and OFD1). The number of families with nonspecific XLMR that have been reported has continued to increase; 58 families or loci are now known. Eighteen new families with nonspecific mental retardation (MRX) have been reported. Two of them, however, were subsequently found to have mutations in the RABGDIA gene, which codes for a GDP-dissociation inhibitor for RAB proteins. In total, 41 more entries have been added to the X chromosome map of XLMR. The total number of known syndromes and MRX families has increased to 178. Of the 120 known XLMR disorders, 53 have been mapped, and 22 have been cloned. Assuming that at least 10 loci are necessary to account for the 58 families with MRX, the total number of XLMR loci counted so far would be 130. Although it is likely that many of the disorders will eventually prove to be allelic, it is not possible at present to determine the precise number of loci for nonspecific XLMR.