Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant neuromuscular disorder. The disease affects specific muscles of the face, shoulder-girdle and upper arm. The biochemical defect underlying FSHD is unknown and there are no specific tests that are diagnostic of FSHD. Genetic linkage studies have mapped the FSHD gene to chromosome 4q35. A DNA marker (p13E-11; D4F104S1, formerly D4S810) has been isolated which recognizes two highly polymorphic loci detectable by EcoRI or HindIII; one locus maps to chromosome 4q35 and shows fragments between about 50 and 320 kb. In FSHD patients deletions occur within this EcoRI/HindIII fragment, yielding fragments that are usually smaller than 28 kb. Characterization of the polymorphic fragments demonstrates that they consist of a 3.2 kb tandem repeat; their number can range between approximately 12 and 96 within the 4q35-specific fragments. In FSHD patients, an integral number of these tandem repeats are deleted, leaving at maximum eight copies.