In several human pathologies, disorders of the epigenetic regulations have been observed. Genetic alteration of modulators of the higher-order chromatin architecture can lead directly to developmental diseases or cancer. Also, the disorganization of the genome can disrupt the equilibrium between gene loci and heterochromatin regions and lead to inappropriate gene expression through position effect mechanisms. A link between position effect and the 4q telomeric region rearrangement is a popular hypothesis to explain the molecular mechanism involved in the FSHD, an autosomal dominant myopathy. Indeed, despite intense efforts over the past decade the gene responsible for this disease has not been identified. Therefore, key to understanding the pathogenesis leading to FSHD is elucidation of how deletion of a tandemly repeated fragment named D4Z4 in the subtelomeric region of 4q35 to a critical threshold causes this syndrome and how flanking sequences cooperate in the pathological process. In order to investigate the molecular mechanisms leading to FSHD, we produced different cellular population stably transformed with GFP gene constructs flanked or not with D4Z4. Reporter genes were integrated randomly or specifically at subtelomeric position thanks to telomeric fragmentation. GFP expression was followed by flow cytometry for up to 100 days. We demonstrated that D4Z4 protects from position effect variegation and telomeric position effect. D4Z4 also blocks the activity of an enhancer when is located between an enhancer and a promoter. Taken together our results suggest that D4Z4 is a strong insulator. Furthermore, the CTCF knock-down lead to a dramatic GFP expression decrease in the cell population containing D4Z4 constructs. The D4Z4 subtelomeric element behaves as a CTCF-dependent-insulator. Furthermore, D4Z4 is sufficient to tether the locus to the nuclear periphery suggesting that insulation properties might be associated with the formation of subnuclear domains. Our goal is now (i) to understand what mediates the transcriptional regulation in term of chromatin architecture and epigenetic regulation and to identify both cis- and trans-sensing effects of the 4q subtelomeric sequences on expression and (ii) to investigate whether D4Z4 protect from repressive structures at the 4q terminus such as the telomere itself or other subtelomeric elements such as the b-satellite-rich region present on the 4qA allele that was reported to co-segregate with FSHD.