We describe a three-generation family in which X-linked mental retardation (XLMR) is associated with minor facial anomalies and brachydactyly. Two brothers and four nephews have "coarse" facial appearance, brachydactyly with widening of the distal phalanges, short stature, and moderate mental retardation. The three obligate carrier women have normal intelligence and normal physical findings. The results of linkage analysis carried out in 1988 using restriction fragment length polymorphisms (RFLPs) were suggestive of linkage to DXYS1 and DXS101 in proximal Xq (Z max = 1.63 at max = 0.0) [Carpenter et al., 1988: Am J Med Genet 43:A139]. The family was restudied with 16 microsatellite loci from Xp11.4 through Xq24. Linkage analysis demonstrated significant linkage to DXS1003, ALAS2, AR, DXS986, DXS990, DXS454, DXS1106, DXS1105, and DXS1220 from Xp11.3 to Xq23 (Z max = 2.53 at max = 0.0). Recombinations detected between MAOB and D X S 1 0 5 5 a n d b e t w e e n D X S 1 2 2 0 a n d DXS1001 place the disease locus between Xp11.3 and Xq23. Among the genes known to map to this region is the XNP gene for the β£-thalassemia/mental retardation syndrome (ATR-X). This fact, along with the phenotypic similarity between our patients and ATR-X males, led us to consider XNP as a candidate gene for this family. X-inactivation studies provided further evidence for the involvement of XNP by showing com-pletely skewed X-inactivation patterns in the three obligate carrier females, a pattern characteristic of carriers of XNP mutations.