Copy-number variation — deleted or duplicated regions of DNA — is widespread in the human genome. A systematic population survey of the common variants provides an invaluable resource for further studies.

What makes people different? Much of the answer comes from inherited differences, and interpreting the extensive variation between people's genomes is a necessary part of understanding the human genome. Variation in the form of single base changes (single nucleotide polymorphisms, SNPs), and repetitive DNA, is already well documented. Adding an extra dimension to human genetic variation is the increasingly evident prevalence and functional importance of copy-number variation. Although most human DNA is present in exactly two copies per cell — one from each parent — some regions can be variably duplicated or deleted, leading to population variation in the number of copies inherited by different individuals. In a Nature paper that has just appeared online, Conrad et al.1 report a working map for frequent human copy-number variation. It is a landmark in providing an unprecedented combination of completeness and spatial resolution, and is likely to stand as a definitive resource for years.