Fig. 1: D4Z4 macrosatellite at chromosome 4q35.

A Ideogram of chromosome 4. The red rectangle indicates the 4q35 region. B Enlargement of the 4q35 region (from A) showing the composition of the D4Z4 macrosatellite. C, D Enlargement of the last four and three D4Z4 repeats. Each canonical D4Z4 repeat is approximately 3.3 kilobases in length, flanked by KpnI restriction sites, and contains exon 1 (orange box), corresponding to the open reading frame (ORF) for DUX4, and exon 2 (pink box). Notably, only the most telomeric (last) repeat in the array is followed by the 3’ UTR-pLAM region (D), which contains complete exons 2 and 3 as well as the canonical polyadenylation signal (PAS). This configuration is essential for the generation of stable, disease-relevant DUX4 full-length (DUX4FL).