Nature Genetics; doi: 10.1038/ng1939; corrected 13 December 2006.

In the version of this article initially published online, the labels 'expression' in Figure 2 panels a, b and d are incorrect. The correct labels are 'activation'. In addition, author Giuseppe Zampino should have affiliation 8 rather than affiliation 7.

These errors have been corrected for all version of the article.