Multipoint linkage analysis of DXS369 and DXS304 in fragile X families.

Diagnosis of carriers of the fragile-X mental retardation gene is hampered by the paucity of tightly linked DNA markers. Recently, 2 new DNA markers RN1 (DXS369) and U6.2 (DXS304) have become available. Both markers are tightly linked to the fragile-X locus, but their location relative to the fragile site was not known with certainty. We have tested these new markers in a multipoint linkage analysis of 26 fragile-X families typed for DXS105 as a proximal marker and DXS52 as a distal marker. Our results establish the order DXS105-DXS369-fra(X)-DXS304-DXS52, which is in agreement with physical mapping results.