AbstractLinkage analysis was carried out in a large four‐generation German family segregating for non‐specific X‐linked mental retardation. Affected males have moderate intellectual handicap. Speech delay, deviant behaviour, and hyperactivity have also been reported. Head circumference and testicular volumes are normal. Cytogenetic analysis failed to show evidence for fragile site or structural abnormality of the X chromosome. None of the obligatory carriers shows any clinical symptoms. Close linkage without recombination (lod scores 1.74 to 2.05) has been found between the disease locus (MRX1) and the polymorphic DNA loci DXS7 (Xp11.4‐p11.3), MAOA(Xp11.3–p11.23), DXS255(Xp11.22), and DXS159 (Xq12) suggesting that the gene responsible for the disease in this family maps in the pericentromeric region of the X chromosome. Linkage data obtained with the flanking marker loci OTC (Xp21.1) and DXS95 (Xq21.2–q21.3) also were compatible with this localization of the MRX1 gene. Close linkage to loci from Xp22, Xq22, Xq24–25, or Xq28 coul