On Dec. 2, 1999, an international team of researchers at the Sanger Centre near Cambridge, England; University of Oklahoma, Norman, OK; Washington University, St. Louis, MO; and Keio University in Japan announced they had succeeded in deciphering the sequence of the 33.5 million “letters,” or chemical components, that make up the DNA of chromosome 22.

This sequence includes the longest, continuous stretch of DNA ever deciphered and assembled. It is over 23 million letters in length. The sequencing of chromosome 22 permits scientists for the first time to view the entire DNA of a chromosome. The results were published in Nature.