The human genome contains the complete set of instructions that codes for life. As we have seen though, exons and regulatory elements that code for proteins make up a relatively small portion of the genome in many eukary organisms. We know that amongst protein coding genes, there are also noncoding and repeated sequences that serve a function of their own. In addition to noncoding rna and single-copy genes, we can also have a variety of different repeated nucleotide sequences such as tandem repeats or even simple-sequence repeats that are repeated over and over throughout a dna sequence stretch. Tandem repeats can be up to several thousands nucleotides in length and be present next to each other in multiple identical or near identical copies. Simple-sequence repeats which are repeats as short as 2 nucleotides. All these different possible sequences that are spread throughout the genome can result in genetic variation both within and across organisms.