As the acronym for VNTR suggests, the variability or differences in VNTR alleles are in the "number of tandem repeats". A tandem repeat is a short sequence of DNA that is repeated in a head-to-tail fashion at a specific chromosomal locus.

Tandem repeats are interspersed throughout the human genome. Some sequences are found at only one site -- a single locus -- in the human genome. For many tandem repeats, the number of repeated units vary between individuals. Such loci are termed VNTRs. One VNTR in humans is a 17 bp sequence of DNA repeated between 70 and 450 times in the genome. The total number of base pairs at this locus could vary from 1190 to 7650.