Figure 2.

Illustration of variable window binding using sequence data from Drosophila melanogaster. The anchor sequence (conserved in all six species) is located downstream from exon 5 and is shown as the 'lower' strand, whereas its binding site is located upstream of the exon to be selected and is shown as the 'upper' strand. There are multiple alternative mutually exclusive pre-mRNA local secondary structures, three of which are shown. The figure is not drawn to scale. (a) Competition among multiple regions for pairing with the anchor sequence provides mutual exclusivity. (b) The binding site when exon 6.25 is selected to be connected with exon 5 corresponds to a window close to one end of the anchor sequence. (c) The binding site when exon 6.48 is selected to be connected with exon 5 corresponds to a window in the middle of the anchor sequence. (d) The binding site when exon 6.3 is selected to be connected with exon 5 corresponds to a window close to the other end of the anchor sequence.