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.
Definition from: GeneReviews
from the University of Washington and the National Center for Biotechnology Information
See also Understanding Medical Terminology.