Clarification of Figure 2

The apparent discrepancies in background and contiguity in the data of Fig. 2, A and C, arise from the limitations in gel processing capabilities due to the large number of samples of each experiment. Thus, it was necessary to collect them in tandem, where some samples were re-run as internal controls in consecutive gels to assure comparable sample handling. Even under these circumstances, it was not possible to obtain the same exact background in all of them, so the figure shows appended images of a set of tandem gels run for the same experiment. This procedure was used for the whole figure.