Correction to: Nature Biotechnology https://doi.org/10.1038/nbt1395, published online 30 March 2008.

In the version of the article initially published, Supplementary Fig. 10 included duplicated images in the row for day 11. As original data could not be located, a revised Supplementary Fig. 10 is shown in the Supplementary Information accompanying this amendment, with the day 11 row removed.