In the majority of facioscapulohumeral muscular dystrophy (FSHD) families (about 95%) the genetic defect has been identified as a deletion of a variable number of KpnI repeats in the 4q35 region, although no specific transcripts from this locus have been isolated so far. Molecular diagnosis is based on the detection by probe p13E-11 of EcoRI small fragments, in the range 10-28 kb, that are resistant to BlnI digestion. In family studies this probe is used with other 4q35 polymorphic markers to assign the haplotype associated with the disease. So far, we performed DNA analysis in 145 FSHD families and identified the 4q35 DNA rearrangement not only in affected individuals, but also in healthy subjects at risk of transmitting the disease, such as non-penetrant gene carriers and somatic mosaics. In addition we applied prenatal tests to 19 fetuses, using DNA extracted from chorionic villi samples (CVS) at 10-11 weeks of gestation. The FSHD status, as determined by the presence of BlnI-resistant small fragments associated with the at risk haplotype, was assessed in nine fetuses; in the remaining 10 cases the disease was excluded. Our results show that molecular analysis of 4q35 rearrangements is a reliable indirect method to perform diagnostic, predictive and prenatal tests in FSHD.