Genes & Development 23: 2812–2817 (2009)
Due to an error during figure preparation for the above-mentioned paper, one of the graphs in Figure 4 was inadvertently duplicated such that the graphs in panels B and E appeared identical. A corrected version of the figure is shown below, such that B remains as originally presented and E is replaced by the correct, nonduplicated graph. This error in no way affects the conclusions of the paper.
The authors apologize for any confusion caused by this error.
Please note that this has been corrected in the online version of this paper.