Subtelomeres are segments of DNA "sub-" or "below" telomeric caps. Telomeres are specialized protein-DNA constructs present at the ends of eukaryotic chromosomes, which prevent them from degradation and end-to-end chromosomal fusion. To conceptually introduce telomeres, introductory biology courses often describe them as a type of chromosomal aglet.
Most vertebrate telomeric DNA consists of long (TTAGGG)n repeats of variable length, often around 3-20kb. Loss of telomeric DNA through repeated cycles of cell division is associated with senescence or somatic cell aging. In contrast, germ line and cancer cells possess a telomerase enzyme which prevents this telomere degradation and maintains telomere integrity, causing these types of cells to be very long-lived.
Subtelomeric chromosome regions are immediately adjacent to the long (TTAGGG)n repeats and also contain repetitive stretches of DNA. In the context of chromosomal analysis, subtelomeres are considered to be the most distal (farthest from the centromere) region of unique DNA on a chromosome.
Recently, subtelomeres have garnered increased attention as research has implicated unbalanced subtelomere rearrangements as a contributing factor in idiopathic mental retardation and other genetic disorders.
Although not drawn proportionately to scale, this image should provide some idea of chromosomal geography.