Sci. STKE, 22 October 2002
The text of the figure from the Perspective by A. Ventura and P. Pelicci was unreadable. A new figure has been created. The online display has been corrected and a corrected PDF is available starting October 29, 2002.
The original, uncorrected PDF can be accessed from http://stke.sciencemag.org/cgi/data/sigtrans;2002/155/pe44/DC1/1.