Genetic mapping of new DNA probes at Xq27 defines a strategy for DNA studies in the fragile X syndrome.
 The fragile X syndrome is the most common cause of familial mental retardation and is characterized by a fragile site at the end of the long arm of the X chromosome.
 The unusual genetics and cytogenetics of this X-linked condition make genetic counseling difficult.
 DNA studies were of limited value in genetic counseling, because the nearest polymorphic DNA loci had recombination fractions of 12% or more with the fragile X mutation, FRAXA.
 Five polymorphic loci have recently been described in this region of the X chromosome.
 The positions of these loci in relation to FRAXA were defined in a genetic linkage study of 112 affected families.
 The five loci--DXS369, DXS297, DXS296, IDS, and DXS304--had recombination fractions of 4% or less with FRAXA.
 The closest locus, DXS296, was distal to FRAXA and had a recombination fraction of 2%.
 The polymorphisms at these loci can be detected in DNA enzymatically digested with a limited number of restriction endonucleases.
 A strategy for DNA studies which is based on three restriction endonucleases and on five probes will detect one or more of these polymorphisms in 94% of women.
 This strategy greatly increases the utility of DNA studies in providing genetic advice to families with the fragile X syndrome.
