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If all the cell's a stage, and the chromosomes merely players, we still don't know whether chromosome 22 has a bit-part or a leading role. But despite its diminutive size — it is one of the smallest of our 23 pairs of chromosomes — we do know it is likely to be quite gene-rich.

So far, for example, chromosome 22 has been implicated in schizophrenia, chronic myeloid leukaemia and trisomy 22, the second most common cause of miscarriages. At least 27 diseases involve some genetic component on the chromosome, which is frequently involved in tumour progression.

About half of its genes, however, have yet to be characterized. Their sheer variety, in terms of differing lengths, is of considerable interest. But the relevance of this won't be known until we have more information from other human chromosomes. So which will be sequenced next?

Greg Schuler, group leader of genome resources group at the National Center for Biotechnology Information, says his short list of candidates would include chromosomes 21 and 7, based on the amount of these sequenced so far (see figure)

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NCBI

One down, more to go: sequencing of the other chromosomes is progressing at varying rates.

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“The emphasis on the draft map of the human genome has delayed finishing these chromosomes a little, and researchers are planning for these next year,” says Schuler, adding that the Sanger Centre itself is projecting completion of chromosome 20.

“Things change quite rapidly,” Schuler says, so it is difficult to be precise about which will be completed next. The chromosomes are at least partially distributed to different labs, and each of these has its own way of prioritizing.

Completion, says Schuler, is “an accident of how [the labs] decide to queue them up”. And what happens when all the chromosomes are complete? “We'll have a big party, of course,” he says.