Nature 444, 481–485 (2006); doi: 10.1038/nature05142 (this issue)

At the request of the Editors at Nature, we wish to clarify some questions that have arisen since the advance online publication (AOP) of our Letter on 23 August 2006. In our Letter, we showed that human embryonic stem-cell lines can be generated from a single cell after its removal from an 8–10-cell embryo. To minimize the number of embryos used, we removed multiple cells from each embryo, and none of the biopsied embryos were allowed to develop in culture.

In our experiments, the isolated blastomeres from each embryo were cultured together in the same medium that was used to culture the parent embryo, and were arranged to avoid contact with each other. Diffusible factors from the other blastomeres present in the media may assist recovery and growth of the blastomere. We have not excluded the possibility that only a subset of blastomeres of an 8–10-cell embryo are capable of forming human embryonic stem cells. These caveats are worth considering for future studies, but do not negate our central finding that blastomeres extracted from an 8–10-cell embryo by mechanical micromanipulation can form human embryonic stem-cell cultures.

We have now added more explicit information on how individual embryos were handled in the form of a table based on Supplementary Table 1 of the AOP version of the Supplementary Information (which has now been removed). This information is now presented in the printed paper as Table 1, to indicate how many cells were individually biopsied from each embryo. In addition, the descriptions for Fig. 4b and d in the legend to Fig. 4 have been corrected (they were inadvertently transposed in the AOP version of the paper).

These clarifications have been incorporated into the paper for the print version and are individually listed as Supplementary Information to this Addendum.