The myopathy facioscapulohumeral muscular dystrophy type 1 (FSHD1) is caused by copy number variation of the D4Z4 macrosatellite repeat on chromosome 4. In unaffected individuals the number of 3.3 kb D4Z4 units varies between 8 and 100, whereas 1-10 units are seen in FSHD1 cases. A homologous and heterogenous D4Z4 array can be found on chromosome 10q, but contractions of this array are typically not associated with FSHD. Discriminating between the chromosome 4 and chromosome 10 D4Z4 arrays, as well as determining the array size, requires the use of pulsed-field gel electrophoresis, Southern blotting, and the isolation of high-quality DNA.