The unveiling of the DNA sequences of human chromosomes represents a new chapter in the unfolding story of genomics, but one with roots in the half-century-old field of cytogenetics. Chromosome-level looks can reveal the specific genes behind certain traits and disorders while providing information on genome organization.

The diminutive chromosome 21--the smallest of the human contingent, despite its number as next to last--is the fourth to be described. Its debut in mid-May attracted attention because 21 is a chromosome with a familiarity factor, in the form of Down syndrome, usually caused by an extra copy of the chromosome. A consortium of 62 scientists, working in Japan, Germany, France, Switzerland, the United States, and the United Kingdom, contributed to the massive effort.1 Chromosome 21's surprisingly sparse gene content has been compared to a desert or even a black hole, calling for a reexamination of total genome size estimates.

The sequencing...

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