Chromosome 20 is the first to be completely sequenced since publication of the working draft in February 2001. An effort of the Wellcome Trust Sanger Center (U.K.), this is the third and largest human chromosome finished to the high quality specified by the Human Genome Project. A paper reporting the work appeared in the December 20, 2001, issue of Nature. Some genes linked to chromosome 20 are implicated in Creutzfeldt-Jakob disease, severe combined immunodeficiency, Type 2 diabetes, obesity, cataract, and eczema. Other chromosomes completed thus far are 22 and 21, which were published in December 1999 and May 2000, respectively.

At almost 60 million bases, chromosome 20 comprises around 2% of the human genome. About 99.5% of its genetically active (euchromatic) regions were sequenced. Using analysis methods similar to those for the other chromosomes, researchers added such approaches as comparisons with newly released genomic sequences, including mouse and the pufferfish Tetraodon nigroviridis. These analyses can help reveal genes and regulatory elements having essential functions that have remained relatively unchanged by evolution. Authors report that some 727 genes were identified in the sequence.

Finishing the sequence of chromosome 20 (covering over 95% of the euchromatic portion with less then 1error in 10 kb) was a difficult task requiring a tedious 'clone-by-clone' approach to find errors as small as single bases. Researchers suggest that some major discrepancies between the public human genome draft and finished data may be due to large duplicated regions of the genome. Completion of the remaining 21 human chromosomes is expected in 2003.