Microsatellites are tandem iterations of simple di-, tri-, or tetranucleotides. They are very useful for the analysis of LOH (loss of heterozygosity) because they are highly polymorphic and interspersed throughout the genome. The microsatellite analysis also allows us to investigate the genetic instability, demonstrated as the expansion or contraction of repeat elements (replication error, RER), which might have resulted from the disruption of mismatch repair genes such as hMSH2 and hMLH1. The microsatellite analysis is a novel tool of genetic analysis which provides valuable information in the field of molecular oncology.