PCR analysis of microsatellite instability.

Microsatellites are simple, tandemly repeated DNA sequences which are abundantly distributed throughout the human genome, and because of their polymorphic nature have been widely utilized as genetic markers (1). They consist of a repeating unit of 1-5 base pairs, averaging 25-60 bases in length, and are commonly found in the form d(CA)n:d(GT)n (2). It has been estimated that there are approximately 100,000 CA/GT repeat sequences in the human genome (3). Studies in colorectal tumors first reported the appearance of instability at microsatellites sequences involving either an expansion or contraction of the repeat sequence (4-6). Such microsatellite instability has now been reported in a variety of different tumor types including lung, breast, stomach, endometrium, and bladder (7-11, reviewed in 12). In addition, a number of other diseases are associated with instability in trinucleotide repeats, such as fragile X syndrome (13), myotonic dystrophy (14) and Huntington's Disease (15).