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The major principles of comparative genomics are straightforward. Common features of two organisms will often be encoded within the DNA that is conserved between the species. More precisely, the DNA sequences encoding the proteins and RNAs responsible for functions that were conserved from the last common ancestor should be preserved in contemporary genome sequences. Likewise, the DNA sequences controlling the expression of genes that are regulated similarly in two related species should also be conserved. Conversely, sequences that encode (or control the expression of) proteins and RNAs responsible for differences between species will themselves be divergent.
(Hardison, 2003) Comparative genomics. PLoS Biol.