Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant myopathy associated with deletions of a subtelomeric repeat (D4Z4). A reduction in D4Z4 copy number coincides with increased expression of neighboring 4q35 genes, implying a normal repressive role for the repeats. Here we examine the effect of increasing D4Z4 repeat number on reporter gene activity in C2C12 cells. Repeat size had only a minor cis -effect on reporter gene activity but greatly compromised myotube formation. This latter trans -effect did not result from expression of a gene within the repeat (DUX4) but likely results from squelching of the D4Z4 recognition complex.
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