 In many species, only a small fraction of the total sequence of the genome encodes protein. For example, only about 1.5 % of the human genome consists of protein-coding exons, with over 50 % of human DNA consisting of non-coding repetitive sequences. The reasons for the presence of so much noncoding DNA in eukaryotic genomes and the extraordinary differences in genome size, or C-value, among species represent a long-standing puzzle known as the "C-value enigma". However, some DNA sequences that do not code protein may still encode functional non-coding RNA molecules, which are involved in the regulation of gene expression.
