Which chromosome was the first to be fully sequenced?

Chromosome 22 was the first human chromosome to be fully sequenced. Researchers announced its sequence in 1999, making it the first complete chromosome-level milestone of the Human Genome Project.

Chromosome 22 was an especially useful early target because it is relatively small and contains many medically important regions. It spans about 51 million base pairs and includes genes associated with conditions such as DiGeorge syndrome, neurofibromatosis type 2 and some cancers. Its completion demonstrated that large-scale sequencing and assembly could produce a reliable human chromosome sequence.

The chromosome’s number can cause confusion. Human chromosomes were numbered largely according to their apparent size under a microscope, with chromosome 1 considered the largest and chromosome 22 originally considered the smallest. Later sequencing showed that chromosome 21 is actually slightly smaller than chromosome 22, but the historical numbering remained.

This achievement did not mean that the entire human genome was finished in 1999. It was one chromosome in a much larger international effort. The Human Genome Project produced a draft sequence in 2000–2001, while later work improved accuracy and filled remaining gaps.