The ends of chromosomes, called telomeres, are composed of a DNA repeat sequence and associated proteins, which prevent DNA degradation and chromosome fusion. We have previously used plasmid sequences integrated adjacent to a telomere to demonstrate that mammalian telomeres suppress gene expression, called telomere position effect (TPE). We have also shown that subtelomeric regions are highly sensitive to double-strand breaks, leading to chromosome instability, and that this instability can be prevented by the addition of a new telomere to the break, a process called chromosome healing. We have now targeted the same plasmid sequences to a site 100 kb from a telomere in a human carcinoma cell line to address the effect of telomere proximity on telomere position effect, chromosome healing, and sensitivity to double-strand breaks. The results demonstrate a substantial decrease in TPE 100 kb from the telomere, demonstrating that TPE is very limited in range. Chromosome healing was also diminished 100 kb from the telomere, consistent with our model that chromosome healing serves as a repair process for restoring lost telomeres. Conversely, the region 100 kb from the telomere was highly sensitive to double-strand breaks, demonstrating that the sensitive region is a relatively large target for ionizing radiation-induced chromosome instability.