We regret that Fig. 1, a colour tip-in, was omitted from the article by H. J. Clarke, S. Varmuza, V. R. Prideaux & J. Rossant in the September issue of Development.

Please attach the enclosed colour plate by the selfadhesive strip so that Fig. 1 faces its legend on page 178 of Volume 104 (1).

We apologize to the authors for this error

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