Addendum to: Nature Medicine https://doi.org/10.1038/8438. Published 1 May 1999.

The Editors would like to alert readers to the fact that two panels within Figure 3a appear to contain features that appear more than once in the same panel. The Editors have been unable to reach the Authors for the raw data and original images or review the original submission due to the passage of time. Readers are urged to take caution when interpreting the content and conclusions of this article.

Due to the age of the article, the original data are no longer available for validation. Readers are therefore advised to interpret these results with caution.