An ultra-conserved element (UCE) was originally defined as a genome segment longer than 200 base pairs (bp) that is absolutely conserved, with no insertions or deletions and 100% identity, between orthologous regions of the human, rat, and mouse genomes. 481 ultra-conserved elements have been identified in the human genome. If ribosomal DNA (rDNA regions) are excluded, these range in size from 200 bp to 781 bp. UCRs are found on all chromosomes except for 21 and Y. A database collecting genomic information about ultra-conserved elements (UCbase) is available at http://ucbase.unimore.it. Since its creation, this term's usage has broadened to include more evolutionary distant species or shorter segments, for example 100 bp instead of 200 bp. By some definitions, segments need not be syntetic between species. Human UCEs also show high conservation with more evolutionarily distant species, such as chicken and fugu. Out of 481 identified human UCEs, approximately 97% align with high identity to the chicken genome, though only 4% of human genome can only be reliably aligned to the chicken genome. Similarly, the same sequences in the fugu genome have 68% identity to human UCEs, despite the human genome only reliably aligning to 1.8% of the fugu genome. Despite often being noncoding DNA, some ultra-conserved elements have been found to be transcriptionally active, producing non-coding RNA molecules. Evolution Researchers originally assumed that perfect conservation of these long stretches of DNA implied evolutionary importance, as these regions appear to have experienced strong negative (purifying) selection for 300-400 million years. More recently, this assumption has been replaced by two main hypotheses: that UCEs are created through a reduced negative selection rate, or through reduced mutation rates, also known as a “cold spot” of evolution. Many studies have examined the validity of each hypothesis. The probability of finding ultra-conserved elements by chance (under neutral evolution) has been estimated at less than 10−22 in 2.9 billion bases. In support of the cold spot hypothesis, UCEs were found to be mutating 20 fold less than expected under conservative models for neutral mutation rates. This fold change difference in mutation rates was consistent between humans, chimpanzees, and chickens. Ultra-conserved elements are not exempt from mutations, as exemplified by the presence of 29,983 polymorphisms in the UCE regions of the human genome assembly GRCh38. However, affected phenotypes were only caused by 112 of these polymorphisms, most of which were located in coding regions of the UCEs.