Effect of target accessibility. This picture shows the predicted structures of target sites sequences for the siRNAs that present the highest error between the experimental and the predicted value of activity. Local secondary structures were computed using the Mfold program. For each structure, an arrow indicates the begin position (right arrow) and the end position (left arrow) of the target site for the siRNA guide strand. Only local structures occurring with high abundance in the predictions issued by Mfold for a same target gene are shown (see Methods). A. Local target structures for siRNA sequences with an underpredicted activity. B. Local target structures for siRNA sequences with an overpredicted activity.