Recurrent somatic mosaicism for D4Z4 contractions in a family with facioscapulohumeral muscular dystrophy

Autosomal dominant facioscapulohumeral muscular dystrophy (FSHD) is caused by contraction of the D4Z4 repeat on 4q35. We describe a FSHD family of unusual genetic complexity presenting with two independent mitotic contractions of D4Z4 in two successive generations. In addition, a non-pathogenic FSHD-sized allele of approximately the same size is interfering with the DNA diagnosis in this family. Interestingly, this allele is not recognized by the probes 4qA and 4qB representing two distal variants of 4qter, suggesting the presence of yet another, infrequent variant of 4qter.