A noncoding polynucleotide sequence between two coding regions (exons) of DNA. Introns are present in eukaryotic cells, but not in bacteria. The length and number of introns in a gene varies, but in some genes introns can contain more DNA than exons. In other genes they are absent altogether. mRNA initially synthesized on intron-containing genes is called heterogeneous mRNA, or hnRNA. While in the nucleus, probably in the nucleolus, introns are removed and the ends of the mRNA are annealed to produce mature mRNA. The latter is then translated into protein. How introns can be removed with such precision, and what happens to them after their removal, is obscure. Several functions have been ascribed to them. For example, they may speed evolution by enhancing recombination between exons. It has also been suggested that they play a role in differentiation. Introns should not be confused with *repeated sequences
, which occur between genes.