Mahua Mukhopadhyay, Marat Gorivodsky, Svetlana Shtrom, Alexander Grinberg, Christoph Niehrs, Maria I. Morasso and Heiner WestphalDevelopment133, 2149-2154.

An error in this paper was not corrected before the article went to press.

The final sentence of the acknowledgements should read: `This research was supported by the Intramural Research Programs of the National Institute of Child Health and Human Development and of the National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health.'

We apologise to the authors and readers for this mistake.