Linkage analysis was performed in a family with non-specific X-linked mental retardation (MRX 15). Hypotonia in infancy was the most remarkable physical manifestation. The severity of mental deficiency was variable among the patients, but all of them had poor or absent speech. Significant lod scores at a recombination fraction of zero were detected with the marker loci DXS1126, DXS255, and DXS573 (Zmax = 2.01) and recombination was observed with the two flanking loci DXS164 (Xp21.1) and DXS988 (Xp11.22), identifying a 17 cM interval. This result suggests a new gene localization in the proximal Xp region. In numerous families with non-specific X-linked mental retardation (MRX), the corresponding gene has been localized to the paracentromeric region in which a low recombination rate impairs the precision of mapping.