The mammalian genome can roughly be divided into two classes of DNA: unique and repeated. Unique sequence DNA occurs only once per haploid genome and codes for structural genes. Repeated sequence DNA refers to multiple copies of particular sequences that are found more than once, ranging from a few (ten or less) to many (greater than a thousand) times in the genome. Depending on the organism, repeated sequences can account for 20-80% of the total DNA. This diverse group can be further subdivided into moderate and highly repetitive elements. The moderately repetitive category is comprised primarily of the tRNA (transfer RNA) and rRNA (ribosomal RNA) genes. In humans, the rRNA genes are clustered on the short arm of only 10 chromosomes, but the tRNA genes are more widely distributed in at least 50 different sites in the genome. These RNA genes are transcribed and the products are used...