Linkage of nonspecific X‐linked mental retardation to Xq21.31

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Abstract
Mental retardation unassociated with the Fragile X syndrome accounts for up to 60% of patients with X‐linked mental retardation. In this investigation, we report on a family with mild non‐specific X‐linked mental retardation (MRX) without other apparent phenotypic abnormalities. Linkage analysis on 27 relatives using 18 polymorphic markers spanning the X‐chromosome demonstrated close linkage to DXYS1 with a peak LOD score of 2.14 at a θ of 0. Numerous families with various types of MRX have now been studied by other investigators using molecular genetic techniques. In addition to the family described in this report, a number of these have demonstrated linkage to the DXYS1 locus. These data suggest that a gene for mental retardation may exist in the region of DXYS1. Alternatively, this area of the X‐chromosome may harbor multiple different but closely linked genes which cause the various types of MRX.