The chromosomal region just proximal to the telomere (end of the chromosome) composed of highly polymorphic repetitive DNA sequences that are typically situated adjacent to gene-rich areas. Microdeletions and subtle rearrangements that disrupt genes in the subtelomeric regions can cause mental retardation; use of fluorescent in situ hybridization (FISH) to evaluate subtelomeric regions is usually required for detection of these abnormalities.