Microsatellites in normal and mutant genes associated with hereditary diseases discussed in this chapter. (A) A normal five exon tandem repeat (TR) gene with 5′ and 3′ UTRs (open boxes), coding region (black boxes), introns (thick grey line) and microsatellites (green boxes) located in promoter, non-coding (UTRs, intron) and coding regions. Copy number variation, due to errors during DNA replication or repair, may result in a modest variation in TR number (shown here as 3 to 4 TRs) that might influence specific regulatory pathways depending on the TR location in the allele. (B) TR expansions result in the diseases discussed in this chapter. The position of each TR is shown (single green box) together with normal (N) allele and mutant (red triangle) length ranges.