In Nature Cell Biol. 3, 918 – 921, 2001. the key for the graphs shown in figure 2b and 2c were incorrect. Please see the correct images below. This errata replaces the previous correction (Nature Cell Biol. 3, 1033, 2001doi:10.1038/ncb1101-1033b). These changes have also been made to the online version of the manuscript.