On December 2, 1999, in the journal Nature, scientists announce that they have sequenced the DNA of a human chromosome for the first time. The genetic sequence of chromosome 22 is one of the first major findings of the Human Genome Project, an international scientific effort to decode the instructions for life hidden in our cells.

Researchers at universities in the United States, United Kingdom and Japan worked together to unravel the sequence of chromosome 22, the second-shortest human chromosome. Each human cell contains 23 pairs of chromosomes within its nucleus, and each chromosome contains a coiled-up strand of DNA. The double-helix of the DNA consists of chemical bases, known by their initials A, C, T and G. Sequences of A, C, T and G create genes, which encode the instructions for making proteins, the building blocks of life.

Chromosome 22, researchers discovered, contains nearly 33.5 million chemical bases and at least 545 genes. Eleven gaps remained, which scientists could not fully decode with available technology. This (almost) complete sequence of a human chromosome was a major step towards unraveling the complete human genome.

The work on chromosome 22 was part of the Human Genome Project, a massive international collaboration to map the entire set of DNA instructions found in a human cell. The project ran from 1990 to 2003. President Bill Clinton called it "one of the most significant scientific projects of all time," with the potential to "promote discoveries that will reduce the burden of disease, improve health around the world, and enhance the quality of life for all humankind."

To obtain the DNA required for their research, scientists ran newspaper ads in Buffalo, New York, to recruit donors. The DNA collected from volunteers' blood samples was distributed in frozen "clone libraries" to labs around the world, "like distributing sets of the same, extraordinarily difficult jigsaw puzzle." The resulting genome research was anonymized, but 70 percent of the genome decoded by the project came from one man, RP11. In 2003, the Human Genome Project concluded by publicly releasing a 90 percent complete human genome sequence. It took nearly two decades for genetic research to fill in the gaps. In 2022, researchers announced that they had decoded the "dark matter of the genome," finally creating a complete sequence. Recalled one scientist involved in the genome project: "[it was] like us being Lewis and Clark. We didn't really have a map."